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.

c) How about the bottom sub-floor? Any movement when you bounce on the
floor?

There is a little creaking in different areas.

d) How do you know what those layers are? Already ripped up a section?

Yes, I pulled up a small area to see what I was working with.

Thanks for your reply.

Dante