The bad news is no, subfloor products won’t make up for an uneven floor. The good news is that I don’t think concrete sticking to the plywood will be a problem. There are two reasons why:
- First, it’s traditional practice to install a layer of tar paper over a wooden floor before pouring a mortar bed as a base for tiles. This means that when you break up the mortar it’ll probably come out nicely in chunks.
- Second, even if the installer did pour the mortar right on top of the wooden floor before installing tiles (highly unlikely), ordinary mortar doesn’t stick to wood very well. A square-mouthed shovel should clean it off nicely. This works well for all kinds of subfloor preparation, especially if you take the time to sharpen the tip of the shovel beforehand. A file or bench grinder does a great job.