Is there any way to borrow an epub from the library that you can then convert in Calibre for use on a Kindle? E.g. "Borrow" it onto an iPad and then somehow transfer to PC for conversion?
Yep, it’s fairly straight-forward to do this; it’s how I borrowed and read ebooks from the library on my Kindle, before buying a Kobo. I’d just delete the book manually once read (as, of course, there’s no longer any expiry date).
All you need to do is import the downloaded book file (which you’d need to download anyway if transferring it to a Kobo via Adobe Digital Editions) into Calibre for doing the other stuff...