You could go this route if you find a really good deal, but have a good portfolio lender on hand (that doesn't require a minimum seasoning period to base the loan amount on the current market value/appraised price as opposed to the most recent purchase price) ready to start the refi when you finish rehab (it wouldn't be very feasible to wait out the minimum 6 month - 1 year seasoning period (most conventional conforming loans require a minimum of 1 year seasoning for a conventional loan) while carrying the hard money loan).

Or is the 10 property mortgage limit the maximum for conforming mortgages only, and an investor could have, say, 20 commercial notes on SFRs if they want to, and still qualify to purchase a property in their own name as long as they have <10 confirming notes?

An annexation history if you will...for example, may be it wasn't annexed until 1995 and so prior to that it belonged somewhere else and there is a chance old records exist elsewhere.Also, let's say it does belong to the unincorporated county in say 1974, where is a good place to look up zoning and building codes during 1974, in order to determine if something would have been legal and conforming back then? 

I suspect the apartment building may be easier to finance than the warehouse, so you also get that out of the equation.Maybe you're not in a zoned area, but if you are, and you're not in conformance, that's going to affect your ability to get financing.  

No bank is going to let you take out an 80% conforming loan knowing the down payment is coming from a 2nd loan (ie owners carry back loan ).

You may have to have offering circulars that conform to specific rules, that are numbered, and you might have to keep track of each of them.

non conforming STR loan How did you add value to the deal?

If the current owner is paying them it likely means they aren't separately metered, which isn't surprising since according to the county tax records this property is not approved for a non-conforming use variance from the zoning board. 

I am obviously not going jumbo loan here and would do two conforming conventionals unless I could get the homeowner to carry a note.

One thing I would want to check is that it is permitted or at the very least "legal, non-conforming."