Wow, Cheryl, what a turn of events. I hope your treatment goes well. Let me share my (non-medical) opinion with you. I spent way too many months on the Big 3 for MAC, and it was no picnic. If I was asymptomatic, there is no way I would ever consider taking them.
I have a friend in chemo for breast cancer right now, and I cannot imagine asking her to add that regimen. She is doing "well enough," but I believe the additional burden would put her flat on her back.

As far as the oncologist's concern about Carboplatin, perhaps you could ask them to consult with your pulmonologist, ID doc, or even NJH and get their recommendation? My friend who had to use high dose prednisone for a long time for an inflammatory disorder was prescribed just azithromycin 3 times weekly to suppress MAC, when I last saw her, it was working for her.

Good luck with your chemo, and whatever comes afterward.
Sue