Doesn't it depend on Play Services? I don't think GrapheneOS could ship it out of the box anyway.

It handles regular SMS just fine on Android. It's great as you can set it as your default messaging app and for regular contacts it sends SMS and for Signal users encrypted messages.

It's iOS where it doesn't work AFAIK as you can't replace the default messaging app.

Yes it does, it integrates very well on Android -- very similar "invisibility" that iMessage has on iPhones.

It does but you will not be able access SMS and MMS messages sent in your prior SMS app in Signal. This has prevented me and based upon multiple threads found while trying to see if it was possible many others from switching to using Signal as my default SMS messenger. It used to be able to import your prior messages but lost the ability to do so and the developers don't seem particularly open to bringing it back.

Yes it does. I installed it out of curiosity and use it as my messaging application. But I think that using it like this, without the secure messaging(since I have no one to send secure messages to) you don't really get any benefit.

Yes.