10% return is probably more than what you are going to get on building a new cabin or buying an existing one these days.  

Window a/c units are all the more reason to keep ceiling fans, as ceiling fans can help a room feel quite a bit cooler. 

Trying to keep an existing tenant often produces more trouble than it's worth.To answer your original question, you should absolutely give them a deadline and a consequence.

I sent out the notices to the tenants that were on month to month leases because you have to honor existing leases. 1 moved out and 4 stayed.

I’d also be open to joining any pre-existing groups who I can add value to.

Do you have any control over what the existing owner can do to the property?

Instead, maybe you could offer to forgive the existing debt if she moves out within 10 days.

Often 10-20-30 degrees cooler than the inland Valley's the OR Coast is a remote but accessible refuge. 

Side tip, if you decide to close without using a agent (not usually a good idea in a new market) then you need to 100% get an inspection (using your own inspector, not the listing agents) and be there during the inspection, title insurance (as always), and have a pre-existing relationship with a property manager.

If you don't want to leverage your existing property, you would need 20-30% down on an investment property loan and it would be at a higher rate than your HELOC.