I want hold shorter term initially.

If the seller declines the offer, it will be for a combination of a) offer amount for the property ($20k) is too low, b) he can't wait 90 days and wants a shorter option period of, say, 30 days max, and/or c) he feels that the consideration you offered for the option contract ("1.5x") is insufficient incentive for him to lock up the property for 90 days.4.

There's always reasons why a longer or shorter holding period might be justified. 

There was a lot I thought we could get done on a shorter timeline, but once I realized that there were bigger more pressing issues that needed to be addressed before we could fix the things I thought needed to be fixed, I realized that would eat up my budget, and kill my time line.

No shorter than 7 day leases per association restriction.  

Ensure that you figure for higher vacancy rates as MF rentals tend to have shorter vacancies.

There are lenders that will do shorter, or zero, seasoning, but they are generally more expensive loans.

I am trying to come up with a scenario/structure that would be beneficial for me in the long run, as far as not taking profits in the immediate future, and beneficial for a partner in the short(er) term.

You also have to be concerned about debt to income ratios as second loans usually carry higher interest rates and shorter terms (think larger payments). 

Obviously, with seller financing, the terms are much shorter.