This link might be of help if looking to try the calibre de-drm route. There is a link about halfway down in the thread in one of my responses to the full instructions:
E-books copy protection prevents imports to LingQ - Support & Feedback Forum - LingQ Language Forums
I’ve personally had good success with Calibre on Kindle books, but I also have an old kindle and downloading via usb I think puts it on an older drm model. I think they have or are eliminated that. So using epubor may be more straightforward to use if that still works.
As for german bookstores that don’t require a German or EU address, I’ve been able to buy from this one:
genialokal - Bücher Online kaufen mal anders
You can even enter your American (or wherever address). Not sure why all the other online German booksellers are such a pain about this when all you want to do is buy an ebook. These books can be bought as epub. I’m not sure about the DRM though. I’ve not tried to import anything I’ve purchased here, or use calibre or epubor to remove any potential DRM on these.