> your broker has plenty of time (2days) and plenty of funds (all of them) to clear your trade
I think you’re thinking about it wrong. All brokers contribute to this posted collateral and those funds, collectively, serve as a fund of mutual insurance in the event of a failure or near-failure of a participant. These are not "partial payment of customer trades, with the rest to come later".
In order to prevent customers from being affected by that near-failure, the clearinghouse first goes to that struggling broker and then to this mutual insurance fund. That is the reason why this must be funded with broker funds rather than customer funds, so the fact that you 100% paid for your shares doesn’t mean that the broker has “all of the funds” required because they can’t use your money to post this. (Otherwise “to protect customers” is not accomplished.)
I think you’re thinking about it wrong. All brokers contribute to this posted collateral and those funds, collectively, serve as a fund of mutual insurance in the event of a failure or near-failure of a participant. These are not "partial payment of customer trades, with the rest to come later".
In order to prevent customers from being affected by that near-failure, the clearinghouse first goes to that struggling broker and then to this mutual insurance fund. That is the reason why this must be funded with broker funds rather than customer funds, so the fact that you 100% paid for your shares doesn’t mean that the broker has “all of the funds” required because they can’t use your money to post this. (Otherwise “to protect customers” is not accomplished.)