Still working on this...
The new version attached sets make-dead-when so that staves with fewer parts have priority over those that they can be merged into. (Disregard what I wrote previously about remove-layers — I got a bit confused.) The engraver now does mainly what it is supposed to, namely provide an intuitive UI for condensing n-parts into shared staves.
Two issues currently, which I am not yet sure how to solve:
1) The engraver is not able to push a change to keepAliveInterfaces to a context that does not already have an event at the current moment.
2) If the status of alive/hidden staves changes on the first moment of a system, the status from the last moment of the previous system will still apply.
Hopefully this is of interest to at least a few others.
divisi_engraver.ly