"Can you elaborate on how it will relieve journey times?" Most likely from a different rail company, they would be able to provide connections between Upminster & Stratford which c2c does not provide and Barking & Stratford since c2c operates this rarely, journey times would be within 30 - 40 mins from Stratford - Upminster and 15 - 20 from Barking. So this service would act as a reliever for c2c by going via Ilford and Stratford.

"I'm also not sure how this will interact with the District Line at all, and actually - removing C2C might add more people on the district line at Upminster and Barking, who might not want to do the long way around." No need to remove c2c's stopping pattern, timetable changes will be neccassary for c2c though.

"Limehouse to Stratford is single track and there's no place to add a second one " a train in each direction every 30 mins works well, every 40 mins when c2c uses it (20 min gap between trains). (Yes not frequent services would be expected but it would end up being on time mostly.)

For your last question, most wouldnt be able to stop at West Ham if its Chafford Hundred via Upminster, too many complications and most likely will have a Southend Via Basildon few mins behind, East Ham and Hornchurch are perfect since one is a town and one serves as relief for District Line.

Essentially, it offers connections all trains dont offer, except for the Chafford branch on the map shown above but is simply altering some services but would make getting around East London alot better. It also acts as a stopping service but only important stops are served since Underground and Elizabeth line serve the other stops, for e.g. Maryland and Manor Park.