To me, the make or break is how hard you think the existing floor removal would be. Your initial description of the existing multiple layers of old stuff sound nasty to me. You might try one spot and see if you can get down to bare cement without encountering any of that glued-down black asbestos papery stuff, and there weren't bad cracks, then you could consider putting the tiles on the slab. To me, with all of the variables you mentioned, plus the insulation issue, I always lean toward a new insulated floor. A plumber/heat guy could give you advise about the heat pipes, I can't see them. As for putting tile on plywood, I have never had a problem. I guess there is a lot of debate over this, but to me its a no-brainer. I have installed or supervised the installation of dozens of tiled floors put on plywood. Like any other tile installation, you need solid framing and subfloor of 3/4" t&g plywood glued and ring-nailed to joists. Then, and here is where a lot of guys start jumping up and down, you put another layer of plywood down instead of cement/wonderboard. This layer should be at least 3/8" and 1/2" or thicker is better. It is glued on and screwed or ring nailed closely. All of the seams must be staggered so they don't line up with any below. You then apply your thinset mortar and tiles and you are done. If this is done correctly then it will be rock solid. If someone uses undersized joists, 5/8" underlayment, 1/4" ply with no glue and doesn't stagger joints then you are in trouble. There is nothing magical about wonderboard or mud jobs. The only place I use cement board is in wet spots like tubs and showers. I think cement board is too nasty to work with and if anything, it is inferiour because it almost always crumbles if handled roughly.