Removing thinset from durarock backer board
For a small area, a grinder with a silicon carbide wheel will clean thinset off, but it is real messy.  For your 320 sq ft area, it could be done, but I would be inclined to pull the backerboard up.

Was the backerboard thinset to underlayment plywood or something like that?

If height is not an issue, you might consider thinsetting new backerboard to the existing, after using a grinder to level out the high spots.

Sorry I cannot offer a more definitive solution; you are in a hard spot.