Order Submitted to Sequencer
Order may be unencrypted or timelock encrypted; submitted to Sequencer, which produces an instant signed reciept including the exact "Tick" at which the transaction was included (90 microseconds granular). Inclusion and ordering is Final at this time.
Orders consumed by execution layer
The "leader" is just another full node that executes the state transition function upon the ordered list of transactions produced by the Sequencer. Since ordering and inclusion is already final, execution is purely deterministic - there is no leader privledge or discretion like other blockchains.
Once the "leader" produces a finalized block, all validators vote on the correctness of the execution of the STF given the ordered tx. list from (1). This step is important for economic finality, but effective finality is reached right after (1). Crucially, there are no fork choice rules needed, as the execution layer can only produce ONE valid block from an ordered list of transactions.