I don't think you can limit pitching changes, as it could lead to injuries. You can certainly require a pitcher face a minimum number of batters (or end of an inning) before being replaced though. Some way to communicate pitches other than signs from the catcher would be useful too, and could lop a lot of time off between pitches.
Yeah, I don't have a big issue with them being eligible, although I don't really care either way. Regardless Ohtani was the same age as normal prospects breaking into the league, so I have no issues with him being considered a prospect.