What is the min amount of AB’s/IP needed to get any benefits for prospects and what is benefit of playing vets versus inj risk?

I think this is a trial and error thing. Me personally, any guys that are young get the majority of the playing time in Spring training. I have my vets start 5 games each and have them pulled in the 4th inning so they usually only get ten at bats or so.

I try to make sure that all of my prospects play in 18 games. I am not certain, but I think development depends more on games played than ABs or IP.

I definitely operate under assumption it is games played - not ABs/IP.
So with settings configured a certain way, you can get a lot of players in a single game.

It's games played. I've seen DITR's get the big ratings jumps with as few as ~6 games played. I put my top prospects as starters, my second tier prospects as rest replacements to sub in +/- 1 run in the 4th inning and my ML starters as def replacements with +1 run in the 7th inning.

Eight tandem starters with all bullpen arms getting 5 pitch count limits. Rest pitchers after ~8 appearances.

This strategy gets just about everyone on the ST roster as much action as possible.