In the UI at least, a new page uses the same master as the previous one.
... although I'd stick to setting the master explicitly, so you know what's going on next time you look at the script. It hardly seems worth the trouble, compared to fussing about if it suddenly
Any specific reason you don't use "app.documents.pages.add()"? That has a documented parameter "reference" -- "Required when the at parameter value specifies before or after. Can accept: Page, Spread, MasterSpread or Document. (Optional)"
Using "pagesPerDocument++" on document preferences sounds a bit ... you know ... lateral thinking.