I think that, regardless of the deal you have with the seller, the primary lender that you refinance with may not like you having a 0% equity stake in the property, or very tiny equity position of less than 25%, and may require you to reduce or eliminate the "junior lien" or whatever you want to call it that is this seller carry item. 

The seller would like to net the most but also carry a note after at least 50k in taxable proceeds, in order to defer her capital gains.

Carry on lol ...............................................

Carry interest of any debt secured by property.I don't believe "necessary" is a word in the statue.  

Ask the seller to carry a note, but I'd buy a SFH instead of a duplex, especially if the duplex needs remodeling!

But was advised to also carry umbrella personal liability.  

Pumps break, lights break, and keeping the deck and any deck furniture cleaned also carries costs.

Quote from @Chris Seveney: @Brian CarberryI wish Vegas would carry betting lines for candidate agendas.

Second lien positions carry more risk since they’re subordinate to the first lien.

While rental ROI appears to be attractive even after carrying costs, the 11% transfer tax when you sell seems like a turn off.