If the $700K sales price is not used correctly to acquire qualified real estate, the exchange could fail, triggering tax liability.Future exchanges will depend on the total value of the replacement property acquired, so entering syndications could limit flexibility.

This tax change could trigger even with offering contacts for those services as it could be construed as contractors.

You would only pay taxes on all passive investments in a traditional SDIRA at distribution, unless you have a mortgage, then the portion of the income derived because of the mortgage will be subject to UDFI (unrelated debt finance income) which will trigger UBIT (unrelated business income tax). 

The underlying note from the lender is still in place and the Due On Sale can be triggered.

Dig into reverse mortgages before pulling the trigger on anything, specifically HECM's (FHA product).

If you can find that deal, pull that trigger.

@Matthew Forrest I just haven't pulled the trigger on making ADU development my priority business plan. 

Selling would likely trigger capital gains taxes (unless you lived in the property for two out of the last five years or use a 1031 exchange).

But if they don't willingly make the payment your only option would be to sue, and they have the upper hand to let the lender know about the sale possibly triggering DOS.

You have three choices...transfer the property and keep the loan in tact hoping they won't trigger due on sale, refinance into a loan that allows it, or simply leave things as they are.