You can leave the reserves blank, but I've had more luck listing each position's backup also in the reserve role. This prevents weird substitutions when the game is an extreme blowout.
I agree. From my experience, when you have 60+ lead, your trainee will come out last 2 minutes no matter the settings. It happened to me already twice this season with trainee given 46 minutes instead of 48. I had +70 and +72 at the end of those matches.
So, put your backup also as a reserve on other positions.