It’s better to keep them as mobi files than converting to epub. Mobi works on almost every device, and converting to epub can always result in messed up formatting or chapters.
If you absolutely have to convert the files to epub for some reason, at least keep the original mobi files as well
Not entirely correct. If you own a legitimate copy of the book on your Kindle you can strip the DRM even on the newest version.
If you acquired the file through illegitimate means and it still has the DRM on it, then the newest DRM is indeed not possible to remove yet