Great post!!
We're about to enter the vacation rental business and I can relate to many of your insights. As far as furnishings we're checking craigslist. I have seen what appears to be decent stuff - now admittedly once we're ready and actually go look at some of it - might not be up to snuff. I've seen listings from free to very cheap as people just want to get it out of their homes. It helps we live near a very affluent area. Craigslist is definitely YMMV.
Another option is the Salvation Army & other thrift stores, which unfortunately my wife has turned me on to! It's now our hobby to swing by these stores periodically to see what's new. We've scored some awesome beach themed pictures, knick-knacks & chairs (need to be refinished) for a pittance. But again all this takes time.
We're going to purchase a "new" (used creeps me out) memory foam bed for our place as well. There are available gel toppers which may be worth considering if we/others find our bed "hot", I may get one and leave it in the closet.
I've also considered the curtain issue. I'll do a 2 layer setup with the room side layer to be cute & window side layer blackout. That's what I like when I travel. In fact the whole cottage is going to be renovated to be fun, bright, comfortable and hopefully a bit quirky. A bit different from what people have at home.
As to reviews - correct me if I'm wrong but don't Airbnb & Homeaway have provisions for an owner response? If someone complained about the curtains I'd put up a response such as "great suggestions we will definitely be incorporating that promptly in our home".
Btw, my wife and I regularly have discussion as to reno & furnishing decisions along the line of "we'd better do this or that just a bit higher end since we're renting it out".