No, apple is not using Google's proprietary RCS they are using Open Source GSM RCS which doesn't go through Google's servers and it doesn't include end-to-end encryption.
Yes, they submitted initial patchset to Linux kernel back in October (1 day after snapdragon X announcement). On Qualcomm's website they say that full support for the chip will be merged into Linux kernel within six months.
You can use dinit-userservd to run services as user services.