I'm a newbie as well, but from my limited time in the Mini 14 world, my answers would be
1)some people are against running steel case through any rifle they own, others are not shy about doing it. I think the consensus is that .223 steel case is handled by the Mini better than it is in the AR
2)Yes, there are a good number of Mini's that have no muzzle device of any kind. It might help with accuracy, as apparently the weight of a flash suppressor/muzzle brake can reduce the effects of barrel whip that occurs as the barrel heats up.
3)The earlier models prior to the 580 series (and some of the earlier 580's) have a thinner barrel, and apparently as it heats up, the groups expand. There are various fixes for that, from barrel supports (like the Accu-strut) to cryo treament, barrel replacement, and in some peoples expert opinion, "buy an AR".