I agree with the others, even when you click your first link, rightmove offers the 321k sold house as a link on the RHS, which was in better nick and had more family friendly space. So even if buyers are clueless about the area, they cannot help but figure out that this house is overpriced.
Only option is to sell this house ASAP for what the market will stand - sounds like it will be in the 270 range if you are lucky, sell the house abroad and buy something they can afford. Although agree that looking at rental options etc is v sensible.
Do not lend them money. DP is well intentioned but nothing good can come of it, believe me. Been there, done that, it will kill your relationship, if not immediately then over the next few years.
Stand firm, OP, this is a tough spot but your instincts are right.