Yeah Denny, you're right, there are really several related issues here regarding the mineral additions:
-Are they necessary for mash enzymatic (or other) activities: No.
-Are they necessary for yeast nutrition during the ferment: Not sure we have a clear answer on this one yet.
-Are they necessary or helpful in contributing to flavor adjustments: Necessary? No. Helpful and effective? Yes.
-At what point in the brewing process should they be added (sparge water, mash water, or boil): Assuming we start with RO water, it sounds like one should add some minerals (calcium) to the sparge water to lower the pH. So if they've been added to the sparge water, is there still a need to add more to the mash or boil? Sounds like, generally speaking, no.
If I may try to summarize the key practical points: Adding minerals to RO brewing water can be helpful in certain processes (such as lautering and maintaining proper pH throughout the process), but may not be essential (if sparge is moderate). But if one makes the decision to add minerals to the brewing water, the most effective place to do so would be to add the pH reducing minerals to the sparge water and the flavor enhacing minerals to the mash or boil.
Did I get that right?
(I'm still curious regarding the question about whether malt in combination with untreated RO water would provide sufficient yeast nutriens.)