Good for you doing your homework!

The problem with some of your questions is the answer is a bit dependant on the environment the enclosure is going in to.

Lets say you get a tub, and thermostat (YOU NEED ONE!) and set it up in a cool room where I am that can be quite cool so lets use my case, 65-68º. The heater and t-stat (thermostat) do the job and heat the surface of the substrate to 90º. The problem is two fold the air temps are very cold and the heater while is heating the ground is not able to heat the air temps as much so lets say the air temp gains 3 degrees that means the so 69-71º. That is quite cool for the cool end temps. This is why I went with a second UTH and t-stat. Many people use a room heater to do the same thing raise the ambient temps in the room to 80ºF.

My case still has an issue in that the air temps are still quite cool and need to be heated as well personally I use two way for that insulated enclosures that keep some of the surface heat and use that to heat air and lighting inside the enclosure (fluorescent) to heat the air during the day at night the temps slowly fall and by morning reach the coolest but then start to lift again.

You can see the problem mine is an extreme example if you had warm room temps high 70s most of the year it is unlikely this would be needed. I can't say for you only for myself.

Never downplay how important ambient air temps are. Ball Pythons need air temps to be at least 78ºF and 85-6 on the higher end to make use of the surface heat coming from the UTH.

why... in case you want to know...

Snakes have a long lung and a short one. The long lung is where the respiration is done the short is not important to this discussion. The long lung passes along beside the heart and liver as well as all the major arteries and veins of the body. If the temp in the lung is the ambient air temp it is very hard to get that blood flow warm enough but running through the muscles on the lower end of the snake that is touching the substrate that is heated.

On a cold day leaning up against a hot car might feel nice but you are still not going to take your jacket off.