Well say someone batting avg is .333 then chances are you will get a hit once every 3 Abs. A guaranteed single will always be better than taking your chances for a XBH.
The counter argument to that is that encourages more challenges which slows the game down even more. Game is already really slow so I wouldn't be a fan of that.
My suggestion is to have all reviews come from the booth much like NCAA football.
I don't think that is conclusive because you actually can't tell when the ball is in the glove because the umpire and runner blocked the view on 2 different angles.