Should teams be allowed an unlimited number of roll on roll off subs?

No. I think teams should be able to sub off players with a head injury without wasting a substitution though.
Impossible to implement though — what keeps one from faking a head injury?
 
I came up with an idea that teams should get 2 free subs, provided that the person coming on is under a certain age and under a certain amount of appearances. So say, must be under 19 at the start of the season and must also not have made more than 15 or 20 appearances for the club at the time game starting.

So for example yesterday, Mou could have brought on someone like Chong (a daft example) but not someone like McTominay who's already got too many appearances, or Pierera who is too old.

Give managers a reason to give kids a chance in a league increasingly hesitant to take a risk because each point is worth so much money.