Some loading manuals will specify a maximum case length and a trim to length. Don't let your cases exceed maximum, but the minimum isn't very critical.
I would guess that being a new loader you just didn't use enough force to seat your primers. Horse em in there until they are at least flush, like someone said I don't think I would do it on a loaded round, but in your normal loading sequence, don't be afraid to use enough force to get them down flat. Primers aren't little grenades, I've loaded hundreds of thousands of rounds, and have only had a couple go off. No big deal as long as you keep the mouth of the case pointed in a safe direction, the same as you do the muzzle of your rifle.
Chambering a round with a high primer is dangerous however, as they could go off, and now it's not just a speck of priming compound but also powder in the round, and worse, the whole thing is contained in the guns chamber, but the breech may not be fully closed. Bad news!! It happens very very rarely, but the potential is there.