1. Dear *****, you are worrying unduly here. One spouse of a married couple has no liability for the debts of the other spouse. Each is a separate entity so far as the law is concerned. So you cannot be held liable for your fiance's debts even if you get married. So, your reasoning is flawed when you consider this to be the case.
2. The answers to your questions are 1. The bank will first try to sell the house. However, the bank cannot touch your assets as you are a separate person in law to your fiance, even if you get married. 2. The bank cannot take assets you might have as a married couple. They can only seek recovery from your fiance's assets, not yours. 3. If you discharged the unpaid debt, you would not then own the house. You would merely have a right to a larger share in it. However, that would only be an equitable interest which you could only realise upon a sale. In the meantime, the asset would continue to be owned by your ex and his former partner. So, you should instead attempt to buy the asset and then discharge the debt. But out your fiance's former partner.
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