You want the “how” to be complete, reasonable, consistent, and flexible.

The main reason I ask is because that is significantly below primary residence mortgage rates let alone any investment loans, if you can get rates like that I might need to make some changes on my end hahahTo answer your original question, assuming all else is equal long-term fixed rate debt is valuable. 

And yes, my reasoning is most of the existing duplex properties are in areas I would never live in being a single mom.

You can offer whatever you think is appropriate and message the seller with your reasoning.

To determine what to offer on a rental property:Determine reasonable market rent, NOT the highest!

I got along well with them.Some reasons for delay are valid, and I've tried to be patient.

I enjoy tenants turnover ever few years for the reason fair market rents tend to increase significantly year over year. 

Know this first:  the #1 reason that landlords fail is their failure to manage to the lease (American Apartment Owners' Assn). 

Is there a demand clause where they can call the loans for no reason?

The only reason I got an LLC was to buy a commercial property because it was a requirement of the lender.