Since an executor/personal representative has not been appointed by the court and no Trust is in place, there is no one who can sign for the deceased. 

You’ll want to have your numbers in order—this means understanding how much the property could sell for, what repairs might be needed, and what kind of offer makes sense for you.When you do contact the homeowner, be upfront and personable.

Hey Alex, sounds like you've got a solid set-up planned for the new duplex! 

There is likely a phone number and persons name that will come up, that you can reference, since he is having bills that arrive to the mailbox and packages to the door with his name on them.You could make it a pre-condition to the seller that he allow a background check on the tenant before closing and your offer is contingent on your approval.Neighbors typically know what's going on in the neighborhood and will know who the guy is and if he is a trouble maker.He probably has a car and a private detective can run the plates.And so on.The information is readily available is the knowledge is worth knowing.

I own an AirBNB in Denver and I just got a year long request and the person doesn't have any reviews.

Arbitrage is not looked upon fondly here.Speaking personally, if I were a property owner with a rental and you said you wanted to do a STR with it, I would not be interested. 

Would it be a better idea to invest in a single family home with only my personal capital, or partner with a colleague(s) and go 50/50 or 33/33/33 on a larger multi-unit property?

If you can't keep control of your personal finances, you are highly unlikely to succeed in real estate investing.

circumstances.. education.. personality .. innate abilities.. lack of understanding basic math..desire .. drive.. familial pressures..  

Do you want to close in your personal name or in an business entity?