So if I want to trade on the exchange, I first need to buy BKC which I then can *consume* in my trades, correct?

Now the other side: Someone applies for a custodianship on the exchange. BKS shareholder approve the proposal and grant the custodian 1000 BKC. The custodian now sells those BKC (in a centralized way?) externally to the exchange and passes the revenues back to the shareholders in form of BTC. Correct?

Is there an incentive to act as a signer? Will they get compensated (through a part of the tx fee in BKC) for their availability and reputation?