Sort of, but the President could create them at will. It took an Act of Congress to expand the regular army, but the volunteers' rank structure was kind of whatever the President wanted.Was brevet or volunteers restricted as well?
My understanding (which may be flawed) is that brevet promotion as a regular is a promotion in the army not matched by the slot you occupy in the regular regimental establishment (at the time; they later became honorary). Since the problem here is a lack of regular army general slots, then there's no scope to do it to get around the seniority problems.
Of course, presumably it would be theoretically possible to start promoting people to LG(V).
In June 1864, yes, because he felt he'd never get another command.Didn't Fremont resign?