It would definitely be simpler, however the routing issue still stands. You would need to have a public IP for every VM, which is getting less practical. The MAC-based proxy makes it so we only need one IP and we can worry about the routing within our platform instead.
but it is http! why can't you do virtual hosting on a reverse proxy?
You can after the initial discovery step, the article mentions this. The MAC routing is for the first step where the device is reaching out to try and find a controller and signal it's available for adoption, which uses an IP address at least in the scheme that is relevant for hosted operators. After that initial channel is established, the controller uses it to tell the device what its hostname is, and you can switch to more normal HTTP proxy routing thereafter.
Yep, that's exactly right!
You definitely can once the device has a hostname set in set-inform. The MAC proxy is just for initial adoption where a device may only know the IP. Such as when you use DHCP Option 43.