There was a dispute years ago along similar lines with Meridians running in multiple (8,9, and 10 cars). This was resolved with a Train Manager in each portion.
I think maybe the RMT feel it will erode their position (and current agreements) on Meridians and future Hitachi trains if they sit back over twelve car 360s.
I believe 12 car 360s were run on Greater Anglia with only one Guard (possibly Driver Only in places). Great Northern did have 12 car 365s (and 321s as well once upon a time) until recently with just a Driver, both of which have no gangways between units.
Although they are different companies with different negotiated arrangements, it does weaken RMT's case (a bit like how 12 car Thameslink DOO on the Brighton Mainline weakened their case in the Southern dispute over the role of the Guard).
I can't see them doing well out of it either way - in fact I was very surprised that 360s were chosen for the Corby service and not DOO in-cab-CCTV-ready 379s - although by chance they happen to have not been finished with by Greater Anglia yet.
Silverlink operated 12 car Class 321s back in 2004 with only one guard who was expected to do full revenue across all cars so leaving the rear 8 unmanned at times.
Indeed if 12 EMUs can run to Bedford multiple times an hour what’s the issue with extending DOO to Corby. Would make a lot of sense operationally as on board staff could then be left to look after the passengers in much the same way as OBS staff on Southern.
I would assume GTR via its previous franchise holders going back to BR have DOO agreements in place.
EMT, who are a former Intercity (Corby route being former MML area, not Central) probably don’t have DOO agreements so a new one would need to be sort with ASLEF and RMT.
Yes great Anglia 1000am braintree to London liverpool Street 12 car 321.
But only as far as Witham.
Then driver only to London liverpool Street.
Only remaining Anglia 12 car class 321 at momment retaining a guard.
Network South East set up with agreements of ASLEF and NUR (later RMT) a area of operation for DOO after a long and bitter strike by NUR members.
Thats why you have isolated areas where guards work and get off like the example above.
LTS (later C2C) retained guards under 357 operation on 12 cars, but later did away with the method of working - beating strikes by simply running 4 and 8 car trains vice 12 on strike days!