Pretty sure you could accomplish this in a large physical server or even a huge resource VM (that has KVM passthrough) with some sort of microvm technology? Then that would obviate the need for "multiple cloud instance per coding thread", it would just be a microvm on the large server.

Then again, I'm just the guy running his mouth, and you guys are the ones actually doing the work :)

BTW, looks very polished and thought-through, I may have to still give it a try!