I have no way to move powder into position though. The shower is installed and done.
Foam injection through opening up a few well placed 1" diameter holes in the sub floor OSB from the crawl space side seems the most likely action to stabilize it at this point.
Considering to use only the low expansion type foam, believe it's marketed for use around windows and doors. I have read where the higher expansion foams will actually lift an entire bathtub that's even been filled with water to prevent lifting !!
I think risk of lift depends on two things: type of foam used, and lateral (sideways) cavities for expansion take-up such that the foam has somewhere to go (off to the sides) when it does expand even the low expansion door and window formula. There *should* be adequate air space areas lateral surrounding the target zone since they used no mortar or other substrate whatsoever. However there is the existing and now cured/hardened foam they put in the other day in response to our complaint and it's unknown how far back it goes.