The example you linked is about push notifications in general, nothing specific to the Signal app. If the concern is that your OS is compromised or spying on you, that's not something E2E encryption can protect against, whether it's Signal or any other app.
I don't think so, you could use the official Linux build as far as I know. I think it needs a phone number but not necessarily a mobile device. I might be wrong though.
> Are you against using an Android (or LineageOS) emulator to do so?
1. It's annoying and inconvenient.
2. It's the result of an artificial restriction by Moxie, for which I can't see any good reason, making me suspicious. In my opinion, this is basically an attack on true mobile freedom.
3. I do not believe in a good app isolation of Waydroid, so I would prefer to use as rare as possible. I also do not trust Android too much. And I will have to run two Signal apps simultaneously.