There is no such thing as a 60 day DL for the minors. Once the decision was made in March to option him to AAA and place him on the DL there, that ship sailed. I doubt it would be allowable to retroactively place him on the MLB DL
Options can be rescinded. Here's a few examples:
Obviously Chargois was optioned a while ago, so I'm not sure it would be quite so straightforward, and it would be the 60-day DL rather than the 15-day DL, but I'm also not finding any language anywhere yet that strictly prohibits it. It's an odd area of transactional rules, to be sure! It seems that, as long as their is no dispute about his health and timeline for return, he and the player's union would welcome the MLB back pay and service time (plus freeing up a spot for an additional union member?).
Otherwise, you are correct that there doesn't appear to be a direct mechanism for moving a player from a minor league DL to the MLB 60-day DL. Per Wikipedia:
Minor League Baseball uses a 7-day disabled list for all injuries. Players who are on the 40-man roster but get hurt in the minor leagues are placed on the minor league DL, but not on the major league DL. One problem this poses is that a player who is injured in the minors and who would be placed on the major league 60-day DL cannot be placed on the 60-day, meaning the 40-man roster spot is not freed up.