Honestly I would settle for 5s with options for additional vehicles to make some units up to 7.
You can get rid of doubling up and all the associated staffing and operational headaches.
Yep, that'd be fine by me: The passenger capacity would be significantly increased against existing single sets, compares well against double sets, and the savings in fuel would be significant over the course of a year, and 4 engines per 5-car unit would mean fewer of those (running for fewer hours per day) to maintain and service across the whole fleet.
I doubt the 7-car extension option would ever get taken up, but we could have wistful conversations about it twenty years down the line.
Are there any platform limitations preventing a 7x26 = 182m train in the platforms XC uses?
I don't think so regarding through platforms, but platform utilisation for terminating services at Reading and Newcastle might require a rethink: Through platform space at the latter must be at a premium now that TPE are similarly unable to use the West end bays with their 5-car 802s, and the number of LNER services has increased.
Though if all the Manchester - Reading/Bournemouth trains are extended back through to the South Coast, and only a handful of Reading - Newcastle trains are reinstated then it wouldn't be a regular occurrence at either location.
Given the awful utilisation of space in the Voyagers, I can't see anyone wanting to touch them with a bargepole when they can get IEPs or new flat front units instead.
Indeed. They were a brave venture at the time into the fairly unexplored world of high speed, high performance diesel multiple units, built for a single purpose and have long since been overtaken by improvements in design.