I'll make an attempt to answer.
"1. If a firearm is more than 12" but less than 24", AND less than 4 pounds, would a brace then be 'OK' without it being an SBR?"
-Don't believe it matters. ATF is no longer using worksheet 4999 and taking a stance that if it can be shouldered it's a SBR.
"2. If one were to simply remove the brace, a pistol is then just a pistol, correct?"
-The brace still must be permanently modified so it cannot be re-shouldered.
"3. Would even having a brace then make the pistol illegal if not attached?"
-Yes, if you have a pistol that will fit it...they call it constructive possession.
"4. Is there a good way to use a sling with an AR pistol that creates an alternative should want to avoid any further hassle?"
-Be interesting to see.
This BATF link covers many of the questions