You will get a little variance in temps simply because of the glass. Not sure it should be 5 degrees but whatever works. My AP cages have a 5 degree variance but that is going through 1/2 inch CDPVC. Just temp gun the whole floor after a couple hours and see what it is. If it is 86-88F then you are fine with whatever the setting is on the Herpstat.
And no the foil tape should not smell like anything. It's a metal tape. You must be smelling something else.
And no tape at all inside the cage. If you need to stick the temp unit on the wall or something like that, I would use velcro tape of the strongest you can find and make sure the stuff is cut smaller than the unit. You dont want any exposed tape at all. And generally you just dont want tape period inside the cage.
I just leave my Acurites on the floor. The snakes will bulldoze them and have actually pooped on them a few times but whatever. i just wipe them off with F10SC and call it good.
Sounds like you got the UTH set up good.
As for having a burnt belly, got a pic? If it looks normal but just the belly scales kind of catch a little on stuff, sounds like he is dry. If his belly is red, gooey and looks like a burn, then its burnt. You will know if its a burn. Just mist the mulch nice and heavy and keep it damp for a few days. Will probably wreck the cardboard hides but it should soften his belly scales back up. My retic gets very rough scales from his neck down about 1/3 of his body in the winter on aspen but he also pushes a lot when hungry. I sometimes switch him to ReptiChips and just keep it nice and moist for a couple weeks and he's fine.
But looking at the temp unit, i see a 40% humidity with a 70 degree temp. You need to get that humidity up to 65-75% and that temp up to about 77 degrees.
As for covering the top, i would use tin foil or at the least tin foil wrapped cardboard. The exposed cardboard top is just going to suck up moisture. And i would cover more of the top. Personally i would cover the whole top by using a 1 piece piece of cardboard. Just set the lamp cage on the cardboard where you want it and trace it with a pen or pencil. Then cut it out so the wire frame fits. Then wrap the whole top in tin foil and secure it with tape on the top. I used a cover like that for my 20 gallon and 40 gallon quarantine tanks and it worked great.
Also i would probably cut the hides down a little shorter. Snakes like something that they can feel the top easily with their head so they feel like they are in a short, small place that animals cant get to them.