Let us assume there is a character who cannot obtain a Force Rating (either through species limitations or any other factor) or be Force sensitive. Can a character who is incapable of having a Force Rating willingly forgo the "gift" of a Force Rating in order to buy into talent trees in a career that normally provides one?

Obviously they won't be able to take Force-based talents. But, to avoid conversations about motivation for doing so, let us assume there is reason enough for the player to want to develop trees for Force-based careers.

There is nothing to stop anyone taking a force user specification as per the normal specification rules. However there are several talents (marked with an explosion around the edge) that need the force to use. Most of the other talents can be found elsewhere so I would ask them why they wanted say a warrior/starfighter ace than an AoR pilot?

I actually could see a Marauder or variant melee character choosing to take a lightsaber tree as their second, instead of another brute close combat tree. Tapani noble comes to mind. There's also one (I've forgotten which) that I considered for a bodyguard droid because it didn't have the gunnery talent gateways and actually had a 5pt bodyguard talent.

Yes, you can get the Force-trees without having a Force-rating. You can also get Force-talents without being a Force-user, you just can't use them without a Force-rating.

Also, keep in mind that Force Rating is actually a Force talent That means that you have to already have a Force rating before you use the Force Rating talent. So you can't just backdoor your way into getting a Force rating by buying a FaD specialization.

I believe Sam S. confirmed what people are saying already: You can take s FU Specialization and buy Talents from those trees however you will be unable to use any Talent that requires a Force Rating. Also Lightsabre Talents only apply to the Lightsabre Skill unless you House Rule it otherwise.

There's a lot of very good reasons a marauder would want to take Schii-Cho Knight spec...

Ancient swords and light foils are both 'lightsaber' weapons, and great for non-force users who want to have fun with the tree.

Indeed, sure they can't make the best use out of said trees, but I can see a few reasons why one would want to go a Shi Knight, an Atstu Striker or anything else; Parry in combination with solid class features are a great investment for a combat character looking to deal with perice or exceedingly high melee damage. Provided they are prepared to take talents that won't do anything for them.

The only thing I would say is "why are you learning about this accicant and forbiddion tradiction?" After all, it's a unusial path to take unless one has hold of the fabled saber.