[ALV]

It also would not surprise me if this is a deliberate simulcast though. A technical fault in E is the most likely explanation in this case, but in years gone by, rather than just simulcasting with World during UK daytime NC hours, Ben would've quickly set up in another studio, A for instance, whilst they were broadcasting the debate in Parliament.
Instead now the default whenever there's a problem with the NC, whether it's technical or staff related, seems to be to go straight over to World.

An example from your narrative:
www.youtube.com 

In the past few years I think the usual protocol of handling technical problems is to switch NC's TX from Studio E to their replay server, which airs the previous hour of the NC output. The team then decamps to a studio and when they're ready the director will switch NC's TX to the decamped studio, crashing right back into the presenter's vision.

I do hope this is a temporary simulcast due to technical issues, rather than a pilot of the merger. They've been subtly piloting stuff like simulcasting Radio 5 Live in the mornings and Politics Live in parliament hours...