I would take a copy of the Land Contract and find a Real Estate Lawyer who can go over the documents to see what your options are. It will cost you an hour or two of his time but it is your best course of action.
Every Land Contract is a bit different so someone will have to go over all of the details. But most likely you are going to forfeit your downpayment and your upgrades. To take it a step further (which will definitely require a lawyer), you may have a case to get out of it, with your money back, if there truly is mold.
There was a recent case in which mold was in a home that was sold and the family all got sick. They eventually got there money back but it took about 2 years and a lot of lawyers fees.
The bottom line is your families health is worth more than any amount of money, so do something about it. Maybe even talk to a Credit Counselor or a Bankruptcy Lawyer about your finances. You need to sit down and look at ALL options and make an educated decision so you can get some relief.
I hope this helps, feel free to contact me if you have any more questions.
Did you have a home inspection ? Did the seller provide a sellers disclosure ? Had the home been purchased prior to you where there might be some kind of document trail ? If you forward your address, I can do a history search for you. Good Luck, Steve