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?

Our appraisal came in short, which lengthens the equity to next home move.

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.

Look at lengthening it to 20-25 years.

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 . 

You might be able to get them to lengthen the term if you shorten the amortization, which will allow them to recoup their investment more quickly, but keep in mind that banks usually tie the rate to a corresponding length of a particular index, so your 7 year term might be tied to the 7-year Treasury.

Obviously if I can lengthen the terms with some seller financing if I am only going to be able to get 5 years that would be beneficial.

I would give agents a second chance and I would lengthen my finish line.I felt my first 3 offers ever were shopped by the listing agents so I stood on my little hill and gave the bird to all realtors 15 years ago.