Congratulations on your AP enclosure - you got one of the best "PVC" enclosures on the market.
I can speak to the RHP as I use 6 of them; "on/off" thermostats are fine but "ramping or dimming" are a better option as they gradually increase the power to the RHP and provide a more even heat - instead of full on then cooling down then full on (you get the idea). You may not even need the flexwatt - try without it first. I hang my probe from the middle vents in the rear about 2-3" above the substrate on the "cool" side and dial in the stat that way. I use Herpstat to control mine and have absolutely no issues. (Side note - if you have a Reptile Basics RHP I've heard people say they need to off gas or cycle them before using with their animals - As I said I've heard this, I do not have any RB RHP's - I only use Pro Products RHP's)
Feeding is a breeze - I used to feed in a separate tub but that became a real PITA with 6 snakes - it also led to me getting tagged 3 different occasions moving the snakes with the scent of feeders in the air - and yes, they can smell it even if they're in another room. Since feeding in their enclosure, no more bites!!! Just thaw your feeder, hot air or water whatever your procedure is, open the door and hold it with the tongs...no worries. If you've got a really aggressive feeder, open the opposite door to give yourself a bit more room. You should be just fine.
Good luck!!!!