Maybe a letter from an attorney would add some motivation.

Try and get them to sign if you can acknowledging the letter.

I have sent letters to the last known address, contacted the city and the county & no such luck of reaching the owners.

You'd be able to get a letter of environmental protection saying you did illegal work, got caught, then had to hire a professional to come in an clean up the mess. 

You'll also likely have to write a strong letter of explanation to explain why your properties went into forbearance and why it's not likely you'll go back into forbearance. 

If you ever go to sell or do a sale-leaseback the buyers and lender will be looking at the tenant mix of the center.

I'm a (will be) first time investor looking at potentially buying in upstate NY, think Windham/Hunter, I'm a big outdoors fan so this would be a mixed personal use and ST rental property (thinking AirBnb, VRBO, etc).

The question I have is whether I should send a courtesy letter ahead of time letting the tenant whose lease I won't be renewing know that they need to be looking for new residence as long as the sale goes through, or if that is too much of a risk since things are not finalized yet.

Or, hire a mortgage servicing company some of which would send out foreclosure letters and enforce and start debt collection process etcHow the best way to handle the no insurance coverage issue and tenant not paying property tax once for all ?

There is no such thing as a pre approval letter to purchase MF.