(Talking to myself on this thread!): I think I've figured this out. The directions Moen supplies are for a replacement of an existing valve. I'm doing new work. I should have placed the valve body in the preferred position, plaster ground tight against outer surface of surround material, mechanism projecting 3/4 inch into the shower space, then plumbed the valve in that position. Valve first, then supply lines.
I ended up keeping the lines where they are and using electrician's plastic spacers to bring the plaster ground up against the surround. Now everything seems OK.
Throughout this project I have been tormented by poor and incomplete product directions and pleasant but unhelpful staff at the supplier.