You public key is registered with a kyc progress. Transactions (transfer of shares) is only settled when all parties have a kyc-verified key. Again, centralized chain (proof of authority). Any and all transactions can be reversed and changed by the regulator if needed. The main difference over today is, you have a open, access-to-all (after kyc) and no intermediaries.

Examples for cashing in: payment for order flow (was recently banned in the EU). They litterally sold their trading data of retail clients for money.

Not sure how you think brokers make money, but they get kickbacks from market makers and fund providers, or simply higher spreads. No one works for free.

You want transactions to be easily reversible?

Payment for order: what data was sold (or do you mean the orders as data)?

You mean broker as in a pure broker, not broker-dealer?

What about policing algorithms, for example? Who'd do that and how?

Who provides liquidity? Just natural supply and demand?