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. 

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

Have a 16 unit trailer park that I’m trying to close the owner is willing to hold 25% of the deal. I’m trying to find a lender that will only make me put 5% down because the seller will have a Second position of 25% o...

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

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.

Gold?

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.  

This is important because GOOD hosts know that good and reliable cleaners and handyman can be hard to find, so they treat their cleaners and handyman lke GOLD, which includes referring business to them, paying them quickly and promptly, respecting their time, and taking care of them on the holidays.   

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