Re: how to remove thinset from plywood sheathing
Thinset should be relatively brittle so that it will chip off if you tap at it with the claw of a hammer. I wouldn't recommend a rotohammer for the job because it will likely gouge and severely damage the sub floor. You say that you have to install underlayment to bring the area level with the surrounding areas. What I would recommend is to just worry about ridges and high spots that will cause bulging of the new underlayment, and leave the rest.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!