I like the idea of the due diligence period starting after all of the documentation has been received

If they are impacted and can show you documentation (yes, proof should be required as some will take advantage of this situation), then you should work through a plan with them.

This has nothing to do with COVID 19 as they signed a document and agreed to be out by March 15th back in mid Jan.  

Or is it good enough to document via email that we have agreed to extend her current terms to a month-to-month until either of us notifies otherwise? 

Bottom line - if you have a federally backed mortgage (Fannie/Freddie) - you will get forbearance guaranteed with no documentation needed, no penalty, no interest and not affect on your credit.

Tell him you need another 45 days plus 30 for extra EM OR you can close on it on time for 70% of the contract price.You should keep trying to call the Tenants to interview them over the phone and get any paper documentation you can from the Seller or PM (Bank Statements, Sales reporting, P&L's, etc).I'm not sure I agree that you need to adjust your price just yet, you just cant close it until we get a better picture of what the fallout is going to be here.

Nice for documenting condition before and after a tenant as well. 

Please reach out to [ ] and ask them to submit documentation stating that it's ok for us to unfreeze the HELOC for draws to be made or that you are no longer seeking to payoff the HELOC."

Document the date you have made the transition.  

Sign with one of the online document companies for e-signature and you're good to go.