I'm not too keen on the seller staying for 2 years. 

Property taxes I usually try to calculate myself, but I found that my calculation is too high most of the time than the figure that is given by the seller. 

Has the legal owners been informed of your method of liquidation, are you prepared to settle if you can't assign, what will you do if  the owners decide not to come to settlement, how will you respond to the assignee or sellers in the event of a default, have you considered specific performance suit by assignee or sellers in the event default, have you made full disclosures? 

I've been running the number into the rental calculator and I typically find that the Magic number to offer is nowhere close to the seller's asking price.  

I think I understand the benefits to the buyer/investor when it comes to using seller financing on a deal.

If you are doing the agent thing correctly, you should be talking to at least 30 or more folks per day, previewing 6 to 10 houses, working with buyers, your sellers, other agents, mortgage brokers and appraisers.  

Vegas has been a sellers decent sellers market since the beginning of 2012 and we have been nowhere close to a balance market. 

On the 6+3 property, take a look at what the seller bought for on 8/10/16.

I'd ask your agent's opinion, the seller, and anyone else who might know. 

I'd try and get an extension with the seller.