Ex spouse, life insurance, and mortgage

I recently married. I have a home that is paid off - only my name is on the title with a will in place leaving it to my children (My husband and I have a pre-nup.) We own a vacation home with rights of survivorship. Each of us has a life insurance policy with more than enough to pay off the existing mortgage should something happen. We are listed as single beneficiaries on these policies.

Ok, now it gets tricky. My husband is still on the ex's house title/mortgage. Per the divorce settlement, she had two years to sell the house. It has not sold and it is now going on 4 years. My husband pays the mortgage out of the child support amount he is, by law, to pay. The child support amount is $1000 and the mortgage is $800. So, he pays the mortgage and the ex gets a check for $200. She is (voluntarily) unemployed so cannot refinance the house. Her boyfriend has moved into the house and we assume he assists with other bills.

My concern and questions are threefold:
1) What happens if my husband dies? If my husband dies, I would not want the life insurance I am listed as beneficiary be taken to pay this debt that his name is on. Nor would I want to be held responsible for the debt in his name.
2) If the ex dies, I don't want to be responsible for a third house (that may or may not sell.) We could not afford this.
3) What happens in three years when my step-daughter turns 18? Now, the $1000 child support stops and my husband is still on the mortgage but now the ex has no income (she has no desire to work.)

My husband is being a nice guy. He does not want to force the ex to sell the house in that would probably cause his daughter to have to change schools. While I commend him for being such a nice guy, I don't want to be taken advantage of - or be held responsible for a mortgage if something were to happen. Any advice would be greatly appreciated.

Your husband isn't "being a nice guy". He's doing what the majority of men do. He's just refusing to face hard issues and he's pretending to be a "nice guy" so he doesn't have to deal with unpleasantness. It's a classic male trait (flaw).

Since she did not sell the house within the 2 year period, she is now in contempt of the court order but that does not matter since he's not going to do anything about it.

Think about the other side of him being such a "nice guy" about this towards her. He's being such a "nice guy" to her that he's placing you at risk. Isn't that nice of him to think more of her comfort and convenience than his new wife's?

It's also quite big of you to commend him for his "niceness" towards her while he has a new wife to put first.