So, we all know that, at one point, the Emperor intended to spare His people the trouble of useful, but exceedingly dangerous, warp travel, by slamming His way into the Eldar/Old Ones Webway, and granting Mankind that means of traveling the galaxy; had Magnus the Red not broken the Golden Throne, HE'D be the one sitting on it, rather than Emps, and on.

Here's my quandaries:

1. Could Mankind, even without the Emperor, still try to do this, even if only on a much smaller scale? Fallen Suns give an idea of how a human psyker might try to force open an established Webway portal, allowing for a positive potential conclusion to that adventure; will try to limit spoilers. Using their own psychic might, maybe even in concert with others, to try and approximate the oomph of Eldar scale power, is there really anything stopping an Inquisitor from building a warband, and marching into the Webway, in the name of the God-Emperor of Mankind? Czerak is seeking the Black Library, as is Ahriman, so I'd assume they can find a way into the Webway, even if they can't, then, navigate it to the proper location.
2. Other than Eldar of four stripes, and the daemons that skulk about the place, are there any other known threats to traversing the Webway, that would affect humans? I've read stuff that claims that the Dark Eldar are how they are, in part, from living in the Webway, to hide from Slaanesh, but I don't know what's them living in a strange place, outside the flows of the galaxy, and what's the Eldar, DE included, created a god that is attached to their races' souls?

I've always been interested in this plan of the Emperor, before it went flop, and always liked to imagine that one of the Missing Primarchs (if the project was so hush-hush that Emps didn't tell Magnus, who he planned to utilize, to make it work, maybe he'd "erase" a legion, to cover them up, once they were sent into the Webway) was actually sent into the Webway, with his legion, to scout the realm out, and fight the foes that dwelt within, sort of strengthening the path the armies of the Emperor would follow, in their wake, and police the place, for the betterment of Mankind; what happened to them, after the Heresy, Magnus screwing things up, and such? I don't know. For the purpose of this post, they are neither here, nor there; they just got a mention.

One of the things I've always wished that the Eldar would do, seeing as they are dying, and sort of accepting it, is to give the Webway to a successor race, but I understand why they won't. If they did die out, tomorrow, though, would it be possible for anyone else to follow in their elfy footsteps, and make use of the Webway?

The single biggest problem is that no extant race, even the Eldar, have the means to meaningfully expand the Webway. The actual material, some sort of psy-active material that can exist in more than 3 dimensions, is something no one has the capacity to create. The Eldar can reshape the Webway such that they can can get most places, but they have no means of creating more; what they do amounts to redistribution of existing material. The spiders that repair the walls seem unable to create anew, and what the Emperor created was a substantially inferior stopgap. The material he used required an enormous, constant influx of psychic energy, which would have made it prohibitively resource intensive to do much more than tap into the existing network. In the current age, any passages lost to daemons remain that way; passing through breached ways are horrendously dangerous affairs. Whatever means of creating the Webway died with the Old Ones, and they failed to pass on the knowledge to the Eldar or anyone else.

The Eldar will never cede the Webway to another, even in their dying gasp; the same overwhelming arrogance that led to the Fall will prevent them from admitting that they could fail and be wiped out. For them, their ultimate failure to overcome their own failings is unconscionable, so the thought of a successor would never occur. Anyone trying would be seen as upstarts and usurpers to be spited to the last, which is how they view Humanity.

As for what happened in the current day, The Emperor is using what is left of his will to keep the golden throne operative and hold closed that Webway portal. If he dies or becomes unable to maintain it, the entirety of the daemonic legions will pour through in minutes, turning Terra into a daemon world, covering the Sol system with a permanent warp storm/warp rift, and ripping the heart out of the Imperium. No Astronomicon and loss of all the forces in the Sol system, including the Grey Knights, will cut Humanity into a handful of local empires and numerous isloated worlds, to be easily swept aside by Orks, Tyranids and Chaos; the extinction event of uncorrupted Humanity.

Finally, there are much worse things than daemons in the warp. The earliest fluff collated for second edition included ancient warp horrors rising from the the first extinctions during the War in Heaven; Proto- and Pseudo-Daemonic entities that are to daemons what daemons are to the material universe. There are also Enslavers, which killed most sentient life after they finished the Old Ones and forced the Necrons into hibernation. In the present, any time a single Enslaver is spotted, that world is immediately logged for exterminatus, though some inquisitors will try to fight them if the world is crucial or exterminatus is not immediately available. One Enslaver can destroy a world as completely as warp rift, and threaten an ever expanding perimeter as more Enslavers are drawn from the warp. Worse, Humanity has basically no defense against them; while warp entities, they are not daemonic so most of the usual methods have no effect, are too powerful to be combated by most psykers, and every body you throw at them dies or becomes an enemy puppet. So for the most part, blowing up the planet is plan A, and there is no plan B.

I was sort of under the impression that, while there are "holes" in the walls of the Webway, and wandering monstrosities do get in, from time to time, the bulk of the Webway is "warded", shielded from the warp, itself, and thus things like Enslavers wouldn't be there; not sure what "natural" denizens call it home, though I can imagine some Lovecraftian horrors dwelling in the furthest recesses, forgotten by all beings, until they pop up.

How much of the Webway is passable varies book to book; a couple give the impression that most of it is dangerous to one degree or another, and the Eldar use only a small number of well known passages. Others have the Eldar with free reign as long as they stay away from a few well known breaches and the passages frequented by the Dark Eldar. Again, lack of consistency across authors. The few common threads are that the bulk of the network in intact and reach across the galaxy; that breaches are generally well known and can only be repaired by the spiders; that some portions of the Webway are irretrieveably lost, even to the spiders; and that no one has the knowlege or skill to reclaim it or restore it back to full functionality. Even the Harlequins can merely traverse it. They do not appear to be in the business of reclaiming the Webway; the knowledge passed on by the Old Ones is lost, and how extensive that was is up for debate. Early fluff had the Eldar bend the Webway how they wished like the Old Ones, and the limits were practical rather than technological. Later fluff had them in the same boat as the Imperium, using and maintaining technology they no longer fully understood. Now it has come full circle, as they once were taught by the Old Ones but have lost what was needed for mastery.

In addition to daemons, there appear to be at least a few things wandering the Webway that do not belong there; but major breaches appear to be contained. It is possible an Enslaver or something similar could find its way in, but that is the sort of thing that would have every Eldar faction in a blind panic to destroy it. I don't think there is any current fluff that concerns an incident like that, but I have not kept up too much with Eldar factions.

Is this a fluff question or campaign background? Like the rest of the universe, you can fall back on, "all canon, not all true" to serve a story.

The Horus Heresy novel, "The Path of Heaven" gives an example of the Imperium's attempts to access the Webway, the dangers, and the major group stopping them being the Navigators. It seems like at the height of mankind's experimentation it was possible, meaning it could be again, but even entering the Webway seems to be frought with peril.

Of course the Navigators are going to do everything in their power to stop Humans from getting a safer method of traveling the universe; Not only is their wealth and power dependent on controlling travel, but if something safer suddenly became wholesale across the Imperium the Navigators would be absolutely ****** because they are mutants that are no longer important to the running of the Empire.

Oh yes, like the Dune-analogues they are often based off of, I can imagine the Navis Nobilite doing EVERYTHING in their power to prevent an alternative form of long-distance/intergalactic/FTL travel from becoming possible; as said, their place, their wealth, and even their right to exist, as they are, would all come under fire. It's another example of "how might things have gone if the Emperor hadn't fallen/ascended at the end of the Horus Heresy, or if He hadn't felt a need to keep so many major secrets from His own people? What were His plans for the Navigators?

As said, though, parts of the Webway are derelict, some dilapidated, and some unsafe, while only what's there is available, so there might always be some need for the Navigators, but at a considerably reduced level, which would cut into their bottom line, and hurt their monopoly. I do still wish, though, that a small Human, loyalist organization COULD figure out how to force their way through a gate, and begin to lay claim to some of the tunnels. Sure, the Eldar would be furious, same as they are when one of their Maiden Worlds they have no intention of living on, nor population to need it; it's theirs , and they'll defend it, from all comers, but, like the Tau, sometimes, they may not be able to rally enough forces to fight off whatever assets said agency commits to the "invasion". The Emperor planned to march something into the Webway, and make sure it was held for Mankind's use, so it might be possible, but yeah, I suppose it'll never happen.