So you walk away from a three month rehab realistically making $15 - 20K.Or you lengthen your runway a bit. 

.- Roof and some inside remodeling was not permitted- Some cracks in drywall and flooring/wall joints- Lead based paint on front of home- Electrical wiring hookup outside of home is fire hazard and should be moved (so lengthen metal pole) - Not sure what the metal pole note means, but I was told moving electrical hookup is pricey.- Some termite damage in main house (will need tenting and underground treatment) and shed will eventually need to be replaced due to damage.- Electrical panel not attached to home, so failed 4-point inspectionAs I will be sprucing this place up for Airbnb, it will also need cosmetic work like landscaping and some painting. 

I'd rather lock in profits at 18-25% ROI in 4-5 weeks than take something more complex that requires permitting or engineering approval that may lengthen the project to 8-12 weeks for a higher return.

How you justified giving 60% of the true value of any property can be snake's head rising later on, financing it lengthens that issue of concern.

Even if the neighborhood is a 50-50 mix the room-by-room heat may reduce resale or lengthen DOM.  

Even if you employ a lease/option strategy to lengthen your holding period beyond one year, profit on the sale is still flip profit subject to ordinary, self-employment income taxes.As @Dave Foster said, flips are not eligible for 1031 tax treatment.  

In general, the more money you spend in improvements, it should be lengthening the lease.  30 days free rent is not as significant as a 5th year.  

And in turn lengthen the entire timeline?

You can always pay extra monthly to shorten the 30 year loan if you want to go that route - but you cannot lengthen the 15 year loan without refinancing.

I would try to lengthen the loan or find a way to lessen that payment each month to buffer you from the expenses your going to get which you haven’t accounted for .