I know these threads tend to repeat themselves, and I'm sorry for contributing to that.
Inherently, the problems with XC are:
[1] Insufficient capacity on large parts of the routes
[2] Rolling stock that has high operational costs on a per seat, or even per-passenger basis if including standees
[3] Working practices that, at times, prevent the capacity that is available from being used (the second set being locked out of use issue)
[2] is a very critical part here, because it is a major contributor to [1] not being resolved. [3] is a problem, but not the most pressing one in my view. It is however, relatively easy to resolve.
There are potential solutions, both long-term and short-term, to most of these. However, as actually resolving issues seems something both our current and previous governments are unwilling to do in most policy areas, people end up suggesting route changes, of varying levels of feasibility, in an attempt to alleviate the issues. I don't personally think that is likely to work, but of course we do not have to agree on that!
Potential solution to what I see as the underlying issues:
To [1] in the short term, take on all 221s that are currently off-lease. In the long term, procure new or cascaded 125 mph bimodes* of an appropriate length (around 8/9/10-car voyager equivalent, so 190-240ish metre long), fixed length with SDO fitted.
[2] is the hardest to resolve in the short term, and I can't identify any realistic solutions that could be implemented in the short term, but people with more knowledge of the operation than I might be able to. Then again, if it was easy it would probably have been done already!
In the long term, however, solutions are available. One solution is to reduce per-seat leasing cost by replacing the voyagers with more space efficient and fuel efficient stock. Another solution is to reduce the amount of staff-time (and therefore cost) needed to run a service, mostly by ending double manning for guards on longer trains. Going for fixed-length supports both of those aims, by reducing dead space that can't hold passengers, and ensuring the guard can access the whole train at any time, removing the need for two guards on doubled-up services as no services would be doubled up.
For [3], in the short term returning to the agreement with the catering staff member acting as the responsible staff member in one of the two units, with the guard in the other unit. The short term solution could continue in the long term, but other options like changing the agreed working practices, or switching to full-length fixed-length trains are also available in the longer term, but unrealistic or impossible in the short term.
To me, the fact that all three of the major issues with XC could be resolved by replacing the rolling stock with something more suitable to the XC operation suggests it is probably time to get started on that. There are at least three manufacturers that offer suitable product lines, the Hitachi 80x, the CAF 397/897, and a 125mph FLIRT UK as was offered but not selected for EMR. Other manufacturers may also be able to deliver suitable products. I think the procurement process should be started, potentially allowing for slower delivery to fit the construction around the many other UK train procurements happening at present (Scotrail, Northern, Southeastern, and GWR of the top of my head!).