For your first question, at least get the consent of the owner's (his niece's) guardian. In other words, the current seller has no right to dispose of this house, and the sales contract reached with you is invalid. You have to sign a copy of how much you paid for the house and what the agreement is. This document shall be signed and witnessed by the owner's guardian. Since the guardian has the right to dispose of the ward's property, the agreement is valid. Then, if his niece still refuses to admit the establishment of the sale when she reaches the legal age and sues the guardian for infringement, and wants to get the house back, you can get your legitimate rights and interests back with the original agreed documents.
For your second question, the court doesn't care about your money, which belongs to two situations. You should also sue your friend (seller) for breach of contract fraud, and ask the court to order the seller to compensate your legitimate rights and interests while ordering you to return the house.
In fact, there is a plot to buy in good faith. If this is approved by the court, the owner should pay some compensation when you check out.