Is it permissible for me to demand the remainder of the down payment from the owner of the house that I backed out of buying after discovering defects in it, and is there any sin upon me for doing so?

The seller is not entitled to the earnest money if you back out of the purchase, even if there was no defect, because earnest money becomes due with the contract. It is permissible to buy a house through the bank if the conditions of Sharia (definition)-compliant Murabaha are met, namely that the bank truly owns the house and it enters into its guarantee, then sells it to you without you being pre-obligated to buy it from the bank or your direct dealings with the first seller. Otherwise, it is not permissible.