here's what i think and some feedback:

1. every other day or as soon as it gets dirty.
2. not sure. but here's a few guesses/questions. A) your heat tape wired wrong. is it one long piece that u snaked through the rack? or is it several strips wired together? B) your temp gun is malfunctioning and/or needs to be calibrated. try your temp gun on another controlled hotspot. is it reading that accurately? C) your paper towel substrate is uneven over the tub and heat tape causing temp readings at different heights and angles
3. i don't think so.
4. keep a more stable ambient temp in the room u keep your snake rack. how often do u check your cool side/ambient temps? how long is it between checking temps that u feel the need to turn on the space heater? u could turn on the space heater and keep it on a thermostat so that it self-regulates or turns itself on whenever ambient temps dips below 78.
5. how many snakes do u have and do u isolate/QT the sick ones? when did u separate the first sick snake? i hope u got it treated and didn't keep it in the same rack with the rest or your collection.

additional feedback: i think your snakes needs hides or appropriate sized hides. the big snake u have pictured above is squeezed between the water bowl and side. unless he is trying to cool off, i think it wants a hide to snuggle into for security. similarly the smaller Banana u have is squeezed between the hide and back of tub. it is also looking for security. so a smaller, snuggier hide would help. neither of these BP's are on the hotspot, so they are getting cold (and possibly sick). put appropriate sized hides for them over the hotspot. they will use it as needed.