[Treble-Enabled Device Questions and Answers] CosmicDan:
1) A/B devices also have a thing called "skip_initfs". In older devices, which is indeed A-only, we have the kernel ramdisk in boot partition. But in A/B devices, the boot ramdisk is only for recovery - when booting the system, the system actually contains the initramfs instead and it gets mounted to / (rootfs) instead of /system.

In short, A/B devices have init and ramdisk all in the system partition. This means Treble ROM's for A/B devices can easily have their own initfs, which makes things a little easier.

2) It also needs bootloader (either SBL or ABOOT, can't remember) support for AB, and these are almost never open source.

3) Treble allows OEM's (the hardware, e.g. Qualcomm) and the ODM (the brand, e.g. Xiaomi) to work independently. Treble provides a contract that the ODM and OEM must each pass verification black-box style, allowing independent development without reliance on the other. Best analogy I can think of is how drivers for Windows work - they don't need to know about what edition of Windows or model of PC it is; they just need to follow standards when making their hardware drivers - and if they do they can be sure that it should work with any other software.

Theoretically, Android P GSI should work straight away on a Treble-enabled Oreo phone. Maybe only with minimal changes - still too early to say. But this is the idea of it.

4) Not entirely true. Unofficial Treble (e.g. like we did for Mi A1) allows us to use GSI's thanks to Phh's work. And unlike many other official Treble devices, we have 100% compatibility with GSI's thanks to the fact that that we can fix GSI stuff on our own end. Many Treble devices are not properly "GSI-ready" vendor implementations, a common theme is that they still put essential Camera stuff in their system ROM instead of vendor (Treble verification I guess doesn't care about Camera support, sadly).

Updates from Google directly is a different program entirely; that's only for devices in Android One program.

Treble support with blobs from before Oreo is practically impossible. They need to be either modified and recompiled with the VNDK standards, or a very smart person needs to shim them. Don't ever expect a pre-Oreo device without source code to be Treble compatible - it's a monumental task that basically requires reverse-engineering the proprietary blobs. If you don't find that useful, then those are the breaks - this stuff was only introduced relatively recently. Treble is not a time machine :)

But again: Treble does NOT mean "updates directly from Google". That's only for official Android One devices.

Maybe one day Google will have an official thing akin to GSI. But not today. As it is, GSI - generic Treble ROM's - are the love child of Phh, there is no such thing as official updates directly from Google outside of Android One (and Pixel ofc).

As for your other speculation, it's mostly redundant - apparently, all devices that launch with Android P are required to have Treble. If I remember correctly. If the pre-P device is popular and open enough, then yeah you will get unofficial Treble (like we did with Mi A1). But that's all up to the device community. But just to reiterate one more time - this does NOT mean updates will come directly from Google.

In case you're wondering why the updates won't come directly from Google (and I predict that this will never be the case, outside of Android One program devices) - simple fact is because Android != Google. Google will never force Android vendors to use Google servers or update channel because Android itself is a very open platform; Treble is an architectural change regarding HAL abstraction - not an enforcement of Google doctrine. It'd be absurd if they did pull a stunt like that; would be like GNU saying "hey Ubuntu, Debian, and all you other guys - you have to use GNU update servers now, all your own servers are not allowed".