Your times to command are about right yes. When you first join, I believe you still currently have a 5 year freeze. When you are frozen, you should always bid for what you want because BA can always release you if it suits them to do so. Short haul commands have in recent times gone so junior that there has been a need to release people from freezes after only a year or two yes. That does not guarantee the situation will continue though.
Your question about a quick release from long haul for a short haul command would indeed be possible if BA had the business need. I would imagine BA prefer to release short haul FOs for short haul commands because the training footprint for the command course is smaller. If they still don’t have enough volunteers, they would release long haul freezes. Pre-covid, I flew with a long haul FO who was about to do just that.