thanks for the updates Jess, Part of me thinks if they still have your original snake and it is looking well, and they have offered the beardie (for the vet bill) , then go for it, my sensible side says 'once bitten twice shy'........
One thing to think about though.. I would be emptying your existing enclosure and washing it (and everytihing you had in it ) giving it a good wash with hot water with a splash of bleach. Leave it out in the sun, without the glass sliding doors on it and let it air thoroughly for a couple of days to totally eliminate any fumes or remaining spray on the surfaces that may be hanging about since you sprayed it for mites. You've had such a battle in the last few weeks, i'd hate to see it suffer from residues or fumes.
and its worth noting that the beardie will have a lot more running costs, both financially and time wise, than your snake, and with young children around make sure you keep washing and sanitising your hands.
sorry to appear to be lecturing but I'm a mum of 2 girls your age (one of whom is also named Jessica) and i can't help but be concerned about how you've been treated
Best of luck Jess, keep us posted