Our other investments (stocks/mutual funds, some mineral rights) had lost a lot value. 

From what I understand there was never a hard-set date for the end of that little fire-resistant cancerous mineral, but the newer the house the less likely that little harbinger of death is likely to be found in your home.

Water heaters can go longer if they are flushed and the water supply is not either corrosive or has a high mineral content. 

Granted, because of this, there are a lot of "miners" here so you'll have to be creative and consistent.    

One other thing to consider is the mineral rights.

You can sell mineral rights if you own those rights.Everything is about contracting and rights.

I rent a room out at my house to a friend right now, and another friend of mine rents out 3 rooms in his house to random tenants and still lives there with a covered mortgage payment.

Just a quick thought . . . as long as I have the mineral rights to my property, can I just theoretically dig for oil under the ground?

The real money is the quick investors for the mineral removal.

I would ask questions like a data miner.