> So they are breaking down the blockchain into small Validator groups. You have to know which Cluster you want to contact. That's... a compromise. It's not even comparable with ETH or the internet where you have this massive pool of validators that all participate over the same network.

No, the thing you quoted just says that devnet validators ignore transactions intended for mainnet and testnet.

> Ok, so, if I have a cluster where I know all the Validators, I can collude with them to just ignore any new Validators from coming in?

Yes, but "a cluster" is "the whole blockchain." This is analogous to saying that you know every Bitcoin miner or every Ethereum node.

> If I'm the leader, can I censor a transaction coming from a Validator? Or perhaps I see a transaction that will move the market, can I sneak in a transaction in my own benefit before broadcasting that one to my validators?

If you are a leader, you can censor transactions, and there will be another leader 2 seconds later. This is similar to the situation in literally every blockchain. In Ethereum today, miners are inserting transactions to extract value from traders. You can read about this here: http://mev.wiki/