My understanding is that most lenders will file a mortgage lien on the subject property as soon as the loan is signed so as to preserve their rights, but what if the lender did not do so before the borrower defaulted?

Regulated by the county of Riverside not the city, thus not subject to the restrictions.

If I utilize qualified funds would this investment be subject to UBIT re UDFI?

I was initially told from others I talked with on the subject that it is very complex structure for a single property portfolio and overkill.

I'm moderately experienced in subject-to (had approx 6 over last 9 years), but this will be my first time as a seller with the underlying loans in my name and more of the risk on my credit.

I will look more into the Zillow and Furnished Finder; Airbnb has a few similar listings that are doing very well but are not subject to the 31 day minimum. 

Also the owner of property might have sold subject to your loan.. are you receiving monthly payments. ?

I would not put any energy into this aspect. the only other way to get one is to take title subject to a 30 year note that can happen as well. 

In the process of BUYING a PURE Option to buy a SFH subject to an existing loan (at a lower fixed long term interest rate)Let say the OPTION "Strike price" to BUY the property IF or WHEN the OPTION is EXERCISED is $300K The seller wants $60K to sell us the OPTION.

As for a information to specific grades for neighborhoods, you may struggle to find that as it can be fairly subjective.