Calibre doesn’t natively support reading DRMed files, but there are anti-DRM plugins which are trivial to install. You need to provide a legitimate Kindle serial number for Amazon DRM, as it uses that to de-encrypt the files. When you add the file(s) to your library, the plugin automatically runs as a file conversion. It basically converts it from a DRM-locked .epub/.azw3 to a DRM-free .epub/.azw3 instead. Since Calibre already has file conversions built in, the plugin simply uses that existing system to spit out a DRM-free version of the same file, then it adds that to your library instead.
Also, you totally glossed over the fact that Plex is simply easier for non-savvy people to set up. Plex provides a unified login experience similar to major streaming services, which Jellyfin simply can’t provide; If your mother-in-law can figure out how to log into Netflix on her TV, she can figure out how to log into Plex too.
And the unfortunate truth is that Plex’s remote access is much easier for 90% of users to figure out. It doesn’t require VPNs or reverse proxies at all. You just forward a port and anyone with access can easily see your server. But my MIL’s TV doesn’t even have access to a Jellyfin app without sideloading. Not to mention the fact that I’d need to walk her through actually setting the app up once it is installed, because there is no unified system for logging in. And if I’m not using a reverse proxy for my Jellyfin server, then I also need to walk her through setting up Tailscale, assuming her TV is even capable of using it at all.
Any single one of those hurdles would make Jellyfin a non-starter if I want to walk my MIL through the setup over the phone, and they’re all currently present. And some of them will never be fixed, by design. For instance, the lack of a unified login page is by design, because a unified login would require a centralized server for the app to phone home too. That centralization is exactly what Jellyfin was made to rebel against, so it’s a problem that will never be “solved”; It is seen by the devs and FOSS enthusiasts as a feature, not an issue.
From a FOSS perspective, Jellyfin is a modern marvel. But it’s definitely not at the same level as Plex when you compare ease of setup or remote access. Jellyfin is fine if you’re just using it locally, or are willing to run Tailscale to connect back to your home network. But if you’re looking for true seamless remote access and need to consider the mother-in-law factor, then Plex is hard to beat.