Rebuilt A 833 front oil leak

Hang on; the "snapring" in question is the great big one on the bearing itself. The retainer has to sit more or less flat on the face of the trans, before the gasket goes on. That big snapring has to fit in the recess provided for it. We call it a snapring but the truth is that there's nothing snappy about it, lol. I myself just don't know the proper name of it.

As to your other questions;
#1,The up and down play at the tip of the input gear is normal.
#2, nothing, The seal and the gasket got it covered.
#3, I have never used any sealer on the gasket, not ever, and never have I had a leak there, lol.
#44, If you look at the factory retainer bolts, they have a raised ring on the surface that seats against the retainer. That is usually sufficient to seal the screwholes. At one time, I had sealing washers for those screws, for redundancy. If your bolts are standard off the shelf bolts with lockwashers, then you will need to seal the threads.