I’ve offered seller financing, 30 year with balloon in 7 (hoping that whatever capital gains they would be hit over the time period I can make up to them even more so through interest payments), I still need to send them the break down of how much they will make through this method in total.

That period was called the Great Recession.

The EMD and buyer's due diligence period was to begin after the rehab was complete.Day 2 December 22, 2015 ~ The rehab/demo began.

I would make interest only payments to them during this period and then repay them with a refi.

Most have minimum 30 day rental periods.

Water, for instance, I pay about $175/month to $225/month in NYC, and yet for a SFR in a neighboring town, in Nassau county, they charge $50 - $60 for a 3 month period, or $20/month.

I'm thinking get a renter in the property after the repairs, and the seasoning period refi again against the new ARV and use that money to fund the build for next property.

The past is no guarantee of the future but pick any 9 year period in the past for properties all around you right now, heck, pick the worst even, 2007 to 2016. 

Right now, my margin is large enough that I estimate being able to price it to sell in a short period of time. 

The payoff period for this is probably not worth the investment.