Ok there are a lot of questions here but I´ll try and be as brief as possible.
Basically if you have a balanced team, Let Coach Decide will do everything you want to do.
If you have a starter and a backup that are fairly evenly skilled then they should get pretty even minutes (give or take some factors i.e. stamina)
Let Coach Decide DOES take into account your depth chart (they all do), but it does so to a lesser degree. This means backups get more minutes if they can perform.
The more "strict" you make your depth chart the more time the starter will get.
If you are good you can do everything on Let Coach Decide and players are less likely to be injured, because they are not playing for whole games.
I hope this gets to the point enough.
good luck