you only have to worry about your BP recognizing your hand as food if you dont reach in there often. unless you keep it on a loose substrate, then you dont need to take the snake out of the enclosure. i feed both my boas and my burm in cage without a problem. i also use newspaper, so no worry of the snake eating the substrate. i would have aproblem if i fed them out of the cage, as you dont wanna see any of them after feeding, they look at me like food.

no, you cannot handle your snake for at very least 24hrs after they ate. ideally, you want to wait 48hrs. when my snakes eat a big meal, switch to a bigger size, or just have a nice hefty lump, i give them 72hrs to digest. thats just a thing i do though.

to prevent your snake from consuming substrate, just dont feed it on loose substrate, or just dont use loose subsrate.

all i can say about your parents is dont quit trying. i always wanted one and my mom always said no. i didnt stop and looky now, i got 2boas and a burm lol. just try to show them that their not all that bad, their not slimy. try to show them that your really dedicated to them. and dont ask them to pay for anything. lol that wont work. try to bring them to a petstore, and show them the snakes. try and get an employee to get one out so you can hold it. just be sure to explain that the conditions kept in the petstore is crappy (most are, not all though) and be sure that if you can get one, to get it from a reputable breeder.