I think you maybe should direct your questions to an HVAC contractor and also to a residential generator sales/installation company.

It would take a pretty large generator to power the electric boiler and the rest of the house and I can't help but think that may not be the best approach. Maybe instead a backup propane boiler and a smaller generator? A backup propane furnace and a smaller generator?

Thank you, Elmer, this is still in the through stage. The propane line to the house is (of course) underground, and I don't know how large a heater I can support with that line. And, how would I turn it on if I'm not there? One thing I could do, as a test, is turn off 3 of the 4 heating elements in the boiler (there are separate breakers for each), and see how low the inside temperature gets. Of course, I would do that during a night when I am there!