So, the Grey KNights are my favorite Space Marine Chapter, hands down, and even the changes in their codex, between Daemonhunters and Grey Knights, doesn't alter that (I think the Inquisitors and their stuff got shafted, but the Knights finally became stand-alone playable), but I find myself confused a bit by their portrayal in DH: Daemon Hunter, so I am inquiring of those who might know more than I do.

First, why between the halbred and that crappy sword do Grey Knights start with the sword? It's the same in the TT, but I believe they can switch to the halbred for free. Here, the "upgrade requires Rank 3/8, and I don't know why they have to wait. Grey Knights mount their stormbolters on their arm, to free up both hands, and the halbred gets Tearing if they use it with both hands. The sword, while a potent weapon, only benefits individuals with field effects, such as the Terminators, with their refractor fields. Why not give the Grey Knights their iconic halbreds, and then let them "upgrade" to swords, if they ever promote to Terminators?

Second, when did the Grey Knights become one of the worst kept secrets in the Imperium? I don't remember rules saying people can't know about them, and that whole groups who see them will be liquidated. I do suppose that seeing them probably means seeing Daemons, too, and then it makes some sense, but people do see Space Marines, and know they exist, so if they just see another color of Space Marine, so what? Who are they trying to keep the secret of the Grey Knights from? Because I hate to burst their bubble, but Chaos knows about them.

Okay, that about covers it, I believe. Thanks in advance.

For your first question, I can't really say why the Grey Knights have to wait for the halberds.

For the second question, people have seen Grey Knights and lived to tell the tale. They just didn't know what the Grey Knights were. There are up to a thousand space marine chapters spread across the entire galaxy, and no one really knows all of them. Often when the Grey Knights show up, they are just a nameless space marine. It's only in incidences of potential daemonic contamination or if people learn or see too much that the Grey Knights and/or Inquistion liquidate everything in sight.

It's swords in the TT game - Terminators can swap to halberds for free, but power armoured knights have to pay extra, as they gain no benefit for the swords. It's a fair balancing tool; sword's are dangerous enough for rank 1, and can still be deadly at higher ranks, while the halberd does have some extra kick that should see some limitations in the absence of a Requirement/Acquisition system.

As for the first, second, and probably third rules about Grey Knights, while Chaos in the abstract 'knows' about the GK, not every cultist or sorceror is in the know. And the GK prefer it that way, so on with the mass purging & mind wipes.

Just for reference, one of the adventure seeds for the Grey Knights is actually worded as "No Witnesses." So yes, details of Grey Knights should remain unknown. I think part of the problem is much of the book is written from the angle of the Ordo Malleus, of which quite a few members would be aware of the Grey Knights.

As for the sword, as others have mentioned, its to put it in line with the (new) codex rules for grey knights. Yeah, I don't like it either; my old army in TT is all now using weapons that make em cost a few points more. Worth it of course (hard to beat higher initiative vs. a crappy melee invuln save).

As you suggested, it's far more about Imperial Citizens seeing/knowing about chaos, daemons and the warp.

Standard imperial mandate dating right back to the crusade is that no-one gets to know daemons exist if it can be avoided.

Being present in a Malleus operational area means being exposed to a moral threat, with concomitent risks of insanity, possession or other corruption. The only truly trustworthy exception is the Grey Knights themselves. Everyone else is mind-scrubbed or liquidated if it is practical to do so.