I apologize if that came out wrong. The idea might very well be the best route to go. It's just that over the past 25 years or so I have seen many attempts to save a dollar that cost a buck and a half to do lol.
Here are my two cents. If you have a slab and you want to pour on top of it while keeping the same footprint that sounds pretty doable and could probably save some money if you don't have to change drain lines, run water, heat runs, electrical etc into the slab.
If you intend to tie into the existing slab and run zones of pex across the joint and have the new and old floors end up at the same elevation it still can be done. Some contractors will not want to mess with tying into and raising the elevation of the slabs and will prefer to start from scratch especially if you as the homeowner want them to warranty the finished product.
The critical thing would be to use enough rebar drilled into the old slab and have enough compaction and sufficient footings to make sure the slabs stay where they are without settling. That would make all kinds of problems with the pex.
Hopefully that response came across better.
Just use plain old spray paint in a can. I've done it many many times and seems to stick really nice. Nothing special either I can't even tell you the brand because I have no clue. But as mentioned doing 2-3 light coats helps.
no expert here, but heat doesn't rise. heat radiates in the direction of least resistance (R value).
warm air or water rises because it is less dense than colder air or water. If you don't insulate you will be heating the ground under your cabin and the earth is a very large heat sink $$$. get some info from an expert in the radiant field as far as tube diameter, spacing, water temp, manifolds, length of runs, and so on. it varies on amount of windows (solar) ceiling height and room type (bed, bath, living area, storage etc.). once you pour over the tubing you get to live with it. I did my own Home 15 years ago and got some good advise (wish I would have taken it all)