Concrete – use leveling compound between plywood and cement backer board

Unless you are concerned about an asbestos material under the tile (such as old linoleum squares), I would remove any old material and start clean. If you keep building on top of the old materials, you might need to rip all your doors down to make them fit with the new floor. Conversely, you need to consider how high to make the new floor in relationship to the old floor. Last, while you may still need floor leveler, a better bet is to install a nice new clean level underlayment board such as concrete board, durarock, hardibacker, or other tile board for tile or a nice clean 1/4" sheet of plywood and paper for hardwood.

Yes you must remove all of the thinset - FOR SURE. Thinset is not meant to be exposed and a leveler on top of it will not protect it enough. From walking and moving furniture you will end up with chunks of thinset/leveler and it will just be a mess.

You either have backer board or plywood under the thinset. You have would really grind off thinset in this scenario but you can:

• pry up the first layer of subfloor and redo it.

• try to hit it with a heavy scraper.

• and what I would try first... pound it out. Most thinset will disengage with heavy vibration. Try a hammer or mallet on a small area. Strike the thinset repetitively pretty hard in a circle pattern. See if you get some chunks disengaging after the first few minutes. If this works it get much easier after you start.