Let's see if I can help you out with some points...
Melamine wood is good if you are going to be sure that no water leaks, or that no humidity will be high enough in a specific area that it will distort the wood. Last thing you want is to have a deformed rack. You should also be careful while you'll build your rack, as you don't want to chip some parts, but that's not a major issue.
Regarding the heat mat, get always certified heat mats that turn off rather than turn on (if they happen to have a problem) so it will not cause any major issue with overheating your snake, and remember never have power applied while it is rolled (even the smallest amount) or overlapping other element., it can cause your product to fail.
For your heat tape, make sure always to aply it correctly (always when no tension is on, of course) and to change it when necessary.
If you put the heat tape under the tub, it has a major surface of heating, making it possible to recieve belly heat for your snakes and to warm up the enviroment. If you put it on the back, it will heat up a smaller part of the surface.
It's better if you decide based on how you are going to build your rack.
I hope I helped a little