It is, I know a server which excepts cliemts from 0.14 to 1.2.x Im not sure how they did it, with UIs, buttt, it is possible.
The ViaVersion on Minecraft Java Edition is a masterpiece lol.. You should respect the developers who is working on this for PMMP and give them some patient.
It does work, BUT, be prepared for errors. You'll have a client with some blocks, and another without that blocks. What do you do? You have to make a seperate instance for each one, im pretty sure
depends, you can render a missing tp block texture or just make it so u cant place it cuz lower clients dont have it