Racks are an exercise in balance. They rarely have the same temps on every level. They depend on a lot of conditions to come from the room that houses them. The ambient and cool side temps relate to the room typically they do not do well in rooms less than 75ºF I would recommend the room be kept to this temp or higher. With out exceptional efforts it is not reasonable to expect more than a few degrees increase in ambient air temps.

Cyprus is fine newspaper as well kraft papers coco coir aspen all work well. Paper towels are the only substrate that is know to be lethal if ingested so I would not suggest them.

Rack materials should be highly water resistant, have high ignition temps and should not burn well (at all). Always keep in mind that cleaning is quite a task in a rack. I would suggest bin covers to speed this job. The upper contact surface is hard to clean and exceptionally difficult to sanitize. I also would suggest every heat source have independent control again with cleaning in mind disconnecting a single 'layer' allows disinfection with out shock risk and with out shutting down the whole rack.

the easiest humidity solution is to humidify the room. this allows good ventilation with out sacrificing humidity. Good ventilation is important building a petri dish grows bacteria too fast and can result in severe health issues. The safest is to opt for more rather than less. I use the one hour guide with the water bowl removed and dry substrate the tub should stabilize to the room RH in less than one hour. (one air change per hour) to avoid what in building would be 'sick building syndrome' a space that does not vent properly and grows an overly abundant amount of bacteria.