1, Yes that is a great hatchling tub. I personally prefer custom enclosures but little snakes do better in little spaces save the money buy a tub to start with and when the snake hits 1000 gms you can make the decision of what kind of permeant home you want and can afford. Starting with a 20 gal is ok it is a bit big but if you went 10gal in 5 month you would be buying a bigger one and then a bigger one still as the snake grows A good thermostat is better than 3 tanks.

I usually recommend buying hides once you have seen the snake, for scale they should be small I ideally the snake will contact the hides on three sides at once. Little snakes grow really fast crazy fast so again a cheap plastic container might be better to start with as they tend to out grow hides in months. Up to you I started with the exo terra smallest hide for my first snake he was in it for about a month and then wore it (came out of 2 or 3 sides) for a few more weeks then I got a medium and large and extra large for him.

Yes it is tight snakes generally like to see but to to be seen. Correct temps being first priority but security being second and humidity third. The tight quarters add to security and it is not for long the snake will out grow that first tub quickly but jumping too far ahead and going big straight away can work but takes more care and skill. My opinion is to get the animal established and feeding and starting to trust you before going big with the enclosure. My older snakes are in quite large enclosures but they all spent time in the tub to get used to my home sounds and smells and me, and the way I feed and do things. Then they moved to large open enclosures (10-18 months after arrival) That also spreads out expenses so buying high end gear isn't such a problem. It is easy to say 200$ snake, 10$ enclosure 120$ T-stat and 20$ heat than adding 200$ enclosure in the mix. See my point?

For a thermostat in a plastic tub under unstable room conditions I'd say a proportional one is the way to go entry level ones are 105-130$ there are cheaper ones but they often have issues with the tubs in coolish rooms. If you go on off the cheaper ones I'd highly recommend the tank. In the end there isn't much cost difference. Tank+cheaper t-stat tub+expensive t-stat is close to the same. There is likely to be a lot of debate with this but a search though older posts will prove this out tubs and on off have more issues.

I don't like the accurites they are no better than the others on the market and they are big and clunky to use. I usually recommend the cheapest thermometer with a probe you can get. (I paid 1.07 for mine on ebay) buy 3 or 5 of them and place the together and use the ones that measure the closest the ones that are off mark them and use them for something else. I also like hygrometers that are designed for humidors. They generally are more accurate and can be adjusted when wrong. They all go wrong over time the test (salt test) is easy and accurate so you can correct the humidor ones. I like the western instruments analogue ones the best cheap and very accurate. In analogue dial types a metal card is a must and I'd add a calibration screw on the back is also a must.

For the little tub the 4x5 is fine. NO a human pad is way too hot! for it to be warm to you it needs to be over 96º this is too hot for your snake to properly digest food. Most do not adjust lower than body temps.

Soldering guns work a sharp drill bit will too. Don't push too hard. ventilation is a bit of a personal crusade with me. I like more than less I don't want to use exhaled breath to increase the humidity. I use holes lower on one end and higher on the other with no holes in the lid. typically low holes over the warm end and high on the cool. It promotes good air flow. How many is a bit of experimentation so start with some and you can add more later. This is where it is good to have a week or more to tinker with things. remember ambient humidity is one thing but inside the hides will always be a bit higher.

It sound like that is about it. I might add a scale as weight is a good indicator of growth and general health. A note book or something to track weight feedings and as many other notes as you can think of records can help solve issues later on. I like to use two thermometers one in each hide. You might add a plant or something to generate 'sky' cover little snakes preyed upon by birds so a plant to provide some over growth sometimes helps.