GNOME GitLab is down currently.
You want to use the version from Flathub, because the Debian version (43.1, not 43.9 which doesn’t exist) is very old. Then, for example, you wouldn’t waste time trying to sign into Firefox Sync, which is already removed several months ago.
Apple does not use WebKitGTK.
There is some more work on WebExtensions underway, but you should not expect this to be ready for end users ever. The universe of WebExtension APIs is huge. I recommend you consider this to be a developer feature intended for people who are interested in implementing WebExtension APIs needed by particular extensions. I don’t see WebExtensions ever being exposed to end users unless a substantial number of contributors decide to start working on it.
That 12.8 MB surely doesn’t include WebKitGTK, which is much larger than that. I don’t know how you’re tracking dependencies, but the dependency chain has dozens of packages. Epiphany itself has far more than just four dependencies, and its dependencies of course also have dependencies…