1. When he marries, his will becomes invalid unless it was made in expectation of the marriage (which, from what you say, is clearly not the case). So either he makes a new will or he dies intestate.
2. You must have a right to live in the house, as 50% owner. Consequently, the value of the other 50% (if you took up that right) would be very low. However, that is really immaterial unless it helped you buy out his wife after his death. The soliciotr should advise you on this, but I doubt whether it would help as she would be living in the house.
3. Your £35K loan (plus interest if the charge has been worded correctly) is protected by the charge. If your father makes a will leaving his 50% to his wife then she will take it subject to that charge.
4. Have you & your father made any arrangements other than the charge for repayment of the £35K? Was the loan made for life or until he went into a home? Or was the term of the loan left vague? If the latter, it might be you could demand repayment - ask the solicitor. But you might not want to do that if it would mean your father & you having to sell the house so he could repay you.