The Flathub security rating is useful but too cautious (so many "false alarms" that people ignore it). It is completely independent from the verification though.

Mixing these up makes no sense.

But for sure, officially supported Libreoffice may be more secure than distro-packaged Libreoffice.

Is any of these applications dangerous or a security risk to the system / user?

Likely not more than Distro packages. They pull in dependencies, and code, just like any other app.

Flatpaks are too pain tolerant regarding EOL runtimes. These may have security risks, and many badly maintained apps are using them, and at least KDE Discover doesnt show a warning here.

Create a fork of an app and verify your website with the fork in Flatpak. The system is already broken

True

By doing so, it undermines a reason why we use GPL and Open Source.

Very good points. It is a good security practice to stay close to a trusted upstream though. Browsers for example may have delayed security patches.

And what about apps where the original author does not care, but was brought to Flatpak by a community member?

Same here, if the upstream tests the Flatpak BEFORE shipping the release, it will work and be fast. If they dont, they ship the update, the flatpak is updated some time after that, it may have an issue, the packagers may need to patch something, solve the issue upstream etc.

The thing is that packagers should join upstream, as only integrated packaging gives this inherent stability and speed.

This is not relevant in many scenarios though. Flatpaks allow to securely sandbox random apps, so they are very often more secure.