If I understand correctly, pistol braces are legal, but fore grips of any kind are not?  So, when shooting, the brace cannot be shouldered, which leads to my question of "can the support hand be used to hold the unaccessorized forearm?"  I can't imagine it being illegal, as it is a pistol, and 99.99 percent of pistol shooters use their support hand.

A 90 degree 'vertical' grip is a no go on a pistol. An angled grip is OK on a pistol though.
If the overall length from muzzle to rear of buffer tube is 26'' or more, a vertical grip is OK but the gun is no longer considered a pistol, it becomes a plain ''Firearm''.

Yes, you can shoot with a second hand on a pistol. By definition, a pistol is designed, made and intended to shoot with one hand but nobody shoots like that, so.......

To be clear, 90 degree vfg is legal if used on a gun that is “not concealable” which is left up to interpretation on a case by case basis. ATF has submitted their opinion that if OAL is greater than 26” then it’s not concealable, and it’s likely that a court would go by their opinion (but the court could use their own interpretation!)

Anyway this is relevant because if you were to be found having actually concealed the firearm, then it must by definition be concealable regardless of ATF opinion, which would make it become illegal