Our broker has told us the umbrella market has tightened considerably due to poor loss experiences for residential and all of her client's umbrella premiums have gone up.   

I hear you, but its just a temporary side hustle during the quarantine, no risk of total loss for me.

Has anyone filed loss of use claims through your insurance for vacation rentals impacted by the Covid-19 situation?

Yes, insurance paid for some, though not all, of the loss - it often doesn’t.  

Lending might tighten up (especially commercial/portfolio loans, i.e. if you are buying the properties in an entity), and might need to sell it at a loss if you can't refinance the hard money loan after rehab and tenanting. 

Death, disability, loss of job, separation or divorce, probate, relocation, tax issues, legal problems", ...just to name a few "life events" that can lead to a so-called "lowball over and acceptance".

Class C tenants tend to be very working class, living pay check to pay check and have been disproportionately affected by the current job losses.

Your loss may help offset the recapture tax. 

The Missouri Supreme Court in Ethridge apparently stated that for equitable subrogation to apply, the party against whom the doctrine would be applied (USAA in this case) must have "engaged in fraudulent conduct or committed acts bordering on fraud that created the condition that caused the lender a loss." 226 S.W.3d at 134. 

So I decided to cut the loss, have the hard money lender take ownership of both properties so I could just walk away.