Or Virginia would have realized that they were being played like a fiddle and revolted as a result. Such occurrences are not uncommon regarding plots by men who think they are too clever by half. Seriously, how could you expect Virginia to have fallen for such a bad faith bait and switch?
Because it didn't. A tactical withdrawal in the face of superior force simply does not constitute a recognition that the other side is in the right.
If it didn't constitute such after negotiations, then it didn't constitute such before negotiations. What then were the purpose of "negotiations"?
Sumter capitulated after being attacked. That was not a recognition of CSA sovereignty, simply a military defeat. A negotiated withdrawal from a militarily untenable position would have been no different.
Then why was it Virginia being negotiated with? Why wasn't it the CSA? Presumably if Virginia (then still part of the Union) was being Negotiated with, it was Virginia which would gain some benefit from the negotiations. What was Virginia's benefit for a deal that only affected South Carolina?
Again, you are flailing here. You are trying to rationalize a nonsensical position; That Virginia gains nothing, but Ft. Sumter gets given up to South Carolina because it's "militarily expedient." From your assertions, it would all appear to be a "Homer Simpson" sort of event without any purpose.
Homer: Save a guy's life, and what do you get? Nothing! Worse than nothing! Just a big, scary rock!
Bart: Hey, don't knock the head, man.
Marge: Homer, you don't do things like that to be rewarded! The moral of the story is that a good deed is its own reward!
Bart: But we got a reward, the head is cool!
Marge: Well, then maybe the moral is, no good deed goes unrewarded.
Homer: Wait a minute! If I hadn't written that nasty letter we wouldn't have gotten anything.
Marge: Mmmm... then I guess the moral is, the squeaky wheel gets the grease.
Lisa: Maybe there is no moral, Mom.
Homer: Exactly! It's just a bunch of stuff that happened.
Marge: But it certainly was a memorable few days.
Homer: Amen to that.
[the whole family laughs]
Neither Buchanan nor Lincoln (nor any other living ex-president) recognized the Confederacy as legitimate, and did not negotiate with it.
Lincoln believed that Congress alone could decide issues of union or dis-union.
As for what Virginia could "benefit", again, the answer is preservation of the Union, their political inheritance for which many Founders had fought & died.
Obviously, that was not enough for Virginia's Fire Eaters, determined only on secession, regardless of consequences.