833 Assembly Woes...

The cavity will easily accommodate one roller, maybe even two.
Do you remember installing those rollers?
Do you remember installing the last one?
Using new rollers, the last one has to go in clocked to the gap in the retaining ring, and slid all the way in. It is highly unlikely that it or any of them will be dislodged, especially greased up with a medium-viscosity grease/NOT Vaseline.
I know of no reliable test for an out-of-place roller.
Yes the 1/8th movement at the tip of the input is pretty normal, and if anything is conservative.

After the fact, running a trans with one roller out of place may never be an issue for a streeter. If the trans, under load, is as quiet in all gears as it is in Direct, chances are that everything is copacetic.