The point is that our engine has some old parts of code that are no longer supported by actual versions of compilers like Android Studio.
We still use an old version of Android Studio to build the APK, and can upload that to Google Play.
F-Droid also builds their APK from source to be sure that the APK is really from the open source code and doesn't have unwanted extra code like e.g. spy ware.
We just have to actualise our engine code, so that F-Droid could build the APK again. Unfortunately we don't have the ressources to do so at the moment.
A signature key is a personal thing. We don't give our signature key to others, and F-Droid wouldn't even accept it. They use a different signature to ensure that it's their version. That's not an issue, it's a necessary security feature.It would also resolve the issue with apps from different sources have different signatures