I agree with the concern about it being too risky to modify extra nights.
I've thought about your suggestion to book a full split stay in everyone's name except one spouse and then booking a separate dummy reservation in the other spouse's name for the purpose of making ADRs. Then, once all of the ADRs have been made, promptly cancelling the the dummy reservation followed by adding the dummy reservation spouse onto the split stay reservations. One could even take this a step further and book the dummy reservation to begin several days in advance of the first split stay reservation to virtually guarantee the early ADRs on the first leg of the split stay. Then, to the extent the dummy reservation doesn't cover all of the days, once all of the early ADRs have been made, cancel the dummy reservation and rebook another dummy reservation with a sligthly later start date to cover the rest of the trip.
While one could argue that booking the dummy reservation to start before the beginning of the split stay is a greedy loophole, I agree that the general idea of the dummy reservation is morally justifiable in the sense that, as you say, it's a bone fide effort by an on-site guest to get their 180+10 (and if a split stay doesn't adversely affect making FP+ reservations, then why should it adversely affect ADRs?).
A couple of years ago I had mixed success calling CRO directly and getting 180+10 for ADRs for a split stay. When I initially called at the 180 day mark for my first resort stay, the CM would not let me make ADRs past my check out date on the first reservation. So I immediately called back, and the next CM allowed me to book over the second reservation. But I've heard that this is less likely anymore (somewhere, I seem to remember reading that the CMs' computer system is now like trying to book online and won't allow making ADRs past the checkout day of a first resort stay until the 180 day mark for the second resort).
As much as I love split stays, and as much as I love planning
Disney vacations in general, planning everything involved with a split stay sometimes almost seems like a full time job.