On this website we will give you a definite do NOT put the uth inside. And yes that would be asking for a burnt snake. Forget the UTH with the enclosure you have it would be a bad idea. I don't have a UTH and my snake gets along fine. UTH's are more necessary for snakes like Corn Snakes that in the wild lay on warm rocks or roads or objects. I don't think Ball Pythons depend on them like Corns. Just keep your ambient temps. where they need to be. Which in Celsius I personally haven't a clue.
In my opinion your pet store is (even though some who work there don't know as much) is leading you in the right direction. I think you can trust they got you started right. I use aspen but I change it out once a month. Never had a mold problem yet. Coconut Husk products are little better and can go longer. When you mist do it very lightly and not so much.
As far as feeding goes, yes it's a good thing to be started on young rats. You want to keep feeding weekly for now. And a good measure to go by is the biggest or thickest part of the snake needs to match the thickness of the rat. Or you can use the weight of the feeder equals 10-15 percent of what the weight of you snake. You said 297 grams so stay with rat pups till your snake reaches 350 grams then try and get weaned rats or something around 35-50 grams total. I personally like to try to hit the 10 percent mark. But no more that 15 percent max.
When your snake reaches 500 grams go to small rats. ( 50-89 grams )