The potential issue of 12 car 458 is not about platform lengths/passenger door operation in the context you mention. It's about signal sighting for the driver of any such formation at Waterloo and Portsmouth Harbour. There are only three platforms at Waterloo, and two at Portsmouth Harbour that can accommodate such a formation, and allow the driver to see the departure signal aspect from his/her seated position in the cab. At all other platforms the signal will be so close to, and above the train, that the driver can not see it.
The platform (ramp end) infrastructure at both those stations is very crowded, such that there is little if any space to move the signal gantries any further out, hence the signal sighting issues.
To sum up, three platforms at Waterloo (two of which are not ideally suited to Main Line departures) and two at Portsmouth Harbour, are in theory useable for 12 car 458, but to plan a reliable half hourly service with so few terminal platforms, would be a very risky ask, if one takes in to account potential point & signal failures, which could leave trains with no platform to use, and cause added disruption. But that is for the current operators to decide.