First you have to pay very close attention to timing deadlines since each sale (unless you sell as a portfolio on one contract) will be it's own exchange. 

One of our concerns is that what happens if the buyer does not finance in 6-12 months and is still sending us payments... can the contract be written so that the deadline to bank finance is the absolute deadline?

I also recommend you establish a deadline and a penalty for failing to meet the deadline.When I offer Tenant a renewal, I give them a written letter with their options.

Took forever to get paperwork, once they missed a deadline where I had to get to documents notarized and we had to reschedule.

some states require that you pay the loan in full prior to the auction if you want to keep the house. https://www.nolo.com/legal-encyclopedia/summary-alabamas-for...Reinstating the Loan"Even though Alabamalaw doesn't provide a statutory right to reinstate the loan before the sale, many mortgages, like the uniform Fannie Mae/Freddie Mac mortgage, allow the borrower to complete a reinstatement.Check your loan documents to determine if you get a reinstatement right and, if so, the deadline to complete one.

Have it inspected and give him a deadline. 

in my state the cure deadline is 11 days before the sale. 

Give them a deadline. 

Their lender is now no longer able to make the next deadline and they will be falling out of contract a second time.

The 1031 exchange timelines are clear and hard deadlines.