I know it would not have been ideal, but i think the Penzance to London train should have not stopped at Bristol for passengers to get on or off, as it was booked to do, as was already busy, so not really able to soak up lots more passengers, the TM could have made plenty of announcements before Taunton that train was now non stop Taunton to Reading, and not call Bristol, and would not be opening doors in Bristol even if stop for crew change, and state this over rides any other announcements made earlier in journey, and if need be have longer stop at Taunton for passengers to get off for Bristol, I realise its tough call to leave Bristol in the lurch but it would have avoided all the problems which later transpired, I think the TM needs to have authority, if he sees that his train is busy already and is likely to be swamped at Bristol, to over ride everyone and make call for train to run non stop Taunton to Reading, and stop start of platform, if need crew change, so that his train is not involved in severe over crowding etc.
I think too many people were involved in the decision making, which ended up confusing everyone and caused a meltdown in the end, when it simply could have been decided to run Taunton to reading non stop as booked. And in the big picture of things, it would have saved a heck of a lot of problems on the night.