[mdp2021]

> both may have come from the same root

But they seem to have the same meaning. The identified root seems to be "mṛ", that of mortality, and they may (/may/, with relevance to the content this paragraph) come from it; that one term in Sanskrit in inherited by Greek and then Latin is the «come from» I meant.

> and 'marya' does not seem to mean mortal

So where does it come from? According to some authors, 'marya' comes from "mṛ" just like 'martya', which you state has a different root. Edit: disambiguation: of course 'marya' is used for "young man". But it seems to mean "a "mortal", someone ready (initiated, young man, no more a child) to develop his defined (limited) path of life".