Your best option is a Conventional or an FHA loan. 

Foremost, Proper are 2 big ones.Don't skimp and get the place insured properly.

That will only leave healthcare to be something I will not have direct ownership or control over as I cannot feasibly own a hospital, nor an insurance company.

Insurance actually yes my other subto just renewed on the 10th.

Credit pulls for the same type of loan within a short window (usually 14-45 days) are treated as one inquiry, so your score should be fine.

If you are getting a loan no there is no way to protect yourself from crushing your credit or having a foreclosure against you if you are the borrower. 

We have secured a home equity loan against our home (which we own out right with no mortgage payment).

I would look at the DSCR loan as the first option.By the way, I suspect you are going to be disappointed by any refi appraisals   ADUs typically appraise poorly   JADUs can lower the property value Vs having the space as part of the primary unit.  

The Homeowners Insurance will include Builder's Risk.

You want an investment that pays for itself, including mortgage, taxes, insurance, maintenance, and even some money set aside for a reserve to cover vacancies and capital expenditures.