Brian Butterfield said last year that the reason they don't have young players taking reps at different positions is because they have enough on their plate as it is and they want them to be as comfortable as possible transitioning through the different levels. The guy has a terrific reputation as a fielding coach, so he must be doing something right.
I'm not saying it's "futile" to discuss unlikely scenarios, I just don't think you'll ever see a top young prospect platoon or DH simply out of "necessity." Before they ever consider making Blake Swihart the DH against righties next season, they'll get someone outside of the organization. Same thing with Moncada, who you've mentioned in several DH scenarios over the last few months. It's not going to happen at this stage of their development. In a September pennant race? Sure, but you'd never see it in the beginning of the season.
As far as Benintendi goes, he'll play against plenty of lefties this season. If they are in a pennant race in September and he struggled against lefties all season, then sure, they might sit him against lefties, but it would never happen in "May or June."
As someone mentioned, they'll find Young at bats while resting all 3 outfielders once in a while and in the DH spot, but they aren't going to play Young against every lefty at the expense of Benintendi's development.