Dante M. Catoni wrote:
a) Is the ply firmly attached?

No, there was a drywall screw about every foot.

b) Ditto the "wavy" boards and what do you mean by wavy?

They are not perfectly flat. I don't think the very thin ply was
enough to compensate.

I think you're right. If you put down another layer of ply BEDDED IN MORTAR
you should be good to go. If the plank subfloors are sound (no flex) then
even 1/4 ply should be OK, 1/2 or 3/4 if you want to be sure. If you do
this, you don't want to screw down the ply so tight over low spots that you
deform it, you're trying to get a level surface letting the mortar fill in
low areas. Same technique if you use cement board instead of more ply.

dadiOH