Alot of questions here. If it's mud that's behind that plastic tile (and it could be-- the old tile may have come down, and the previous owner just covered the mud wall back up), the question is how good is that mortar? From what I've read so far, it already needs to be repaired up top, but that's no big deal. The problem is that if it's mud, and you tear it down and go to put cement board back up, you may have to fir the cement board out to get it to cover the edge of the tub, being that most float coats are a MINIMUM of 3/4" thick, and alot of times, they'll be even thicker than that. You need to see what exactly is behind that plastic tile, and how thick it is.