You don't need an office to be visible - plenty of guards at other TOCs do a great job of doing so every day without them.
I'm not sure fitting offices to units would make much of a difference, especially at certain TOCs which are notorious for the apparent shyness shown by some of their supposedly public facing employees.
A very good point and one which blurs the line between Actual DOO and Mock DOO, yes there are guards who do a great job and I'm not mocking them especially as I will and have forwarded praise where due however the grade is not helped and I'm sure you see my point here that there's a minority who simply are not up to the job and let the side down.
It's these guards who aren't as active as they ought to be who make it easier for DOO to be pushed though as it makes it obvious that they are surplus to requirement compared to these who actively carry out their duties and are visible to passengers who prove why DOO isn't acceptable.
The bottom line is office or back cab, the various TOCs and they know who they are ought to push their employees to be more visible.
Predominantly yes but not exclusively; Colchester - Clacton/Walton and Manningtree - Harwich require guards. I've never been able to work out if Ipswich services do or don't north of Colchester, videos I've seen on Youtube imply that they do but Manningtree has DOO cameras.....
Correct me if I'm wrong but when you used to have the Liverpool Street to Peterborough services via Ipswich and yes I did use it once in preference to the direct route from Kings Cross that it was DOO between Liverpool Street and Ipswich with a guard joining the train to continue onto Peterborough.
It was over a decade ago so I might be wrong but I'm sure that's how it used to be?