All 15 players impact your rating - so yes bench players that weren't commons would boost your deck. Whether you get to epic or not is hard to tell due to the number of untrained and partly trained players you have, just check yourself.
The general consensus on the forum here seems to be to play the highest season you can. The reasoning being that just making the playoffs gives you x2 cards from the tier below.
Making the semis in the tier above gives you the same reward as winning the tier below and making the semis is cake. I have never failed to do that and when you get into the final you are guaranteed 2x epics just for making it.
Lastly I would think you should be getting into Epic to try and get some more pro Epics (any legends is a bonus)... I mean why would you be grinding away in UR if you are trying to, or ready to, pro a bunch of legends?