1. Yes your manual is saying you can use steel shot in an 835 X-Full
2. I think it was a typo error. Email Mossberg and ask to be sure.
The "T, F, #2 and BB" are all steel shot sizes. My understanding was that different size shot, particularly the larger T and F, need more open constriction to give similar patterns to smaller shot.
"Pattern" is what we call the group of holes left in a paper target by 1 load of shot fired, generally at 40 yards for testing. It is referred to by shotgunners to show how far the pellets spread at a given distance. A full choke should spread to roughly 40 inches at 40 yards. A modified choke should be close to 40 inches at 30 or 35 yards. Each gun and choke will be slightly different in actual pattern size and density. You choose choke and shot size based on game or target type, and expected distance to the target.
You can't fix stupid....however ignorance can be cured through education!