Can you drop wife at MIA J in the rental car and then return it to the rental car return place? You might be able to arrange for wheelchair assistance to meet her at the curb. Of course, it's easier to do this if there is a third (or even fourth) person traveling with you to stay with her (and someone else to go with you while you return the car to help with bags that aren't dropped off at the curb initially).

BTW, are you flying to Argentina or is it a domestic flight, so that there might be curbside check in and bag drop for your airline at MIA?