IIRC, it’s more than just an API, Apple has implemented device specific signing that isn’t possible to replicate in this way, as far as I can recall. I don’t know the full details but I know this is part of the reason nobody has really even attempted to fly under the radar this way. It’s part of what’s downloaded on fresh installs of the operating system (hence why it sometimes works in a VM, though I’ve heard with each release iMessage is harder to get working in a VM)
Yes, but you could emulate that too. If a Hackintosh can validate itself as an authentic Apple computer, an app that replicates all the same calls to Apple's servers that that Hackintosh makes should be able to do just as well.
I suppose it’s certainly possible. But a Hackintosh operates at a very low level, making macOS believe it is running on supported hardware. It doesn’t need to know how the OS authenticates itself to iMessage. My suspicion is that disassembling that whole process would take a lot of work (and be very brittle, because Apple would probably deliberately break it)
It presumably also integrates into the OS at a deep level: the system that pushes messages to the computer is not iMessage specific, it’s OS-level. So you’d need to reverse engineer that, too.
iMessage needs to support older versions of macOS and iOS, so I suspect someone who cracks however identityservicesd works will have the ability to make this work "permanently".
I'm not very familiar with the workings of iMessage but why can't an Android device copy the protocol and make the relevant API calls directly?