How? It's not a MitM or anything like that, it's connecting exactly how an Apple device would connect. Everything is still E2EE, just one of the ends can now be an Android device.
LinuxSBC
Vivaldi does too. It's nice.
From what I understand, their guess is that Apple is now checking if the device also has support for other services, such as FaceTime. Beeper Mini and pypush don't pretend to support FaceTime, so it breaks.
Their hope was that they got close enough to an actual Apple device that breaking it would break Apple devices. It turns out they weren't close enough, but they could be with a few improvements.
Because they can't break that. It's using real Macs, so if they break iMessage for Beeper Cloud, they break it for their customers.
Yes. They have a fork of Synapse that they can continue to use even if the license prevents them from using upstream (which doesn't seem true, but I could be wrong).
Kind of, but it's more complicated. I'm not sure if the app itself will be open source, but currently, the method they use is. Either way, the hardest part is already done, but you still need a client (maybe; they might open-source it) and a notification server. I'm planning to attempt to build a Matrix bridge if I have enough time and it's not beyond my skills, but if you don't want the messages to be decrypted by the server, making the notification server and maybe client would be really difficult.
Intentional ineptitude resulting from malice is still malice.
That's to prevent multiple entries by one person. Their security is very good, with audits and their products being largely open source (for this, PyPush. For Beeper Cloud, their Synapse fork and their bridges.). Only the parts that don't matter to security (the clients, mostly) are closed source.
No, they know that a message has been received, but the phone is what decrypts the message. Beeper can't see it.
And that's what they're doing.