I don’t understand the problem with having signal here on Fdroid. It doesn’t violate the guideline. For example we have Newpipe on Fdroid which is not allowed on the Google play store because google doesn’t accept a third party app for youtube. BUT it is shipped in the Fdroid repository.
So why not just make a Signal fork, removing the copyright stuff so that we don’t get problems, and just connect to their servers. I don’t see the problem and don’t understand why for certain apps it is accepted and others not.
It’s a shame that Signal doesn’t want them to be on Fdroid. It’s not in the spirit of free software. I still use it as a compromise, but it makes me sad.