Please see sketch:
http://i899.photobucket.com/albums/a...wall-floor.jpg

I'm concerned that if I install the XPS and stud wall per this sketch,
I'll be left with a depression underneath these two items due to the
thinset and tile I want to install.
My other option is to install thinset and tile first, but then I run
the risk of damaging the tile floor with all this stud wall
construction taking place over it.
3rd option is what "Bill" suggested below: to make a sill under the
XPS and stud wall similar in thickness to thinset&tile. But my
concern is that may spall as it's so thin and so narrow.

Opinions appreciated!