I would guess that this is not a problem as long as the DH does not take a position on the field. Regarding the substitution, I am guessing the second pitcher entered the game for the 1B who left the game. The fact that two pitchers are on the field should not be a problem. If this is a problem, the Angels may need to rethink their efforts with Cowart and Walsh.
I don't see why MLB would have a problem with this, except for the Red Sox loss.