Exactly - much more time/space at the Swansea end (rather than dealing with conflicting movements at the Manchester end). Plus, to match demand, you'd go to Manchester in the morning and south in the afternoon.
If you are going to run loco-hauled (with all the complications/ expense that this means having four/more coaches. And if you are going to run four/more coach trains then that means the Manchester duties.
But the capacity problems at Piccadilly would be a nightmare to hunt locos around (and only allow Wales & Borders to use a few platforms), so you either need to loop it (like I suggested) or get some DVTs.
You're talking sense for the Manchester end, the reason I said it was pointless is because you'd need DBSOs/DVTs at the Swansea end anyway (or you could have another loco attach at the other end in Swansea as Anthony suggested, but that needs an extra loco).
But in the long run there will be modern DMU's released by the spread of elecrification.
More 158s would help yes, but I doubt we can see any of them until 2015 at the earliest. 170/171/172 door arrangment is wrong for most ATW services except the ValleyLines, Swanline, Cardiff - Cheltenham and perhaps a Cardiff - Hereford/Shrewsbury all stations service if such a thing is started someday. Regardless, there will be events such as festivals where a long train will be needed as an extra. The 2 ex-WAG mark 2 sets would be great for that, but not sure ATW would keep some in reserve for that sort of thing if they weren't also acting as spares for a few more mark2 sets operating in day-to-day service.
Modern in the sense that they will have passenger operated doors and access for wheelchairs. Both of which are a no no for Mark 2's.
I think most trains have passenger operated doors, certainly the mark2s. You probablly mean power-operated doors. As for access for wheelchairs, I think that's much easier to solve than the lack of power-doors. DBSOs and BSOs both have double doors for luggage don't they, a wheelchair should fit through those. Providing an accessible toilet might be difficult though. As for WAG not funding it, they fund Gerald which isn't great for disabled passengers either, but an improved service for able-bodied passengers.
I don't think you have considered the implications of access for the disabled. My own station has just had X million pounds spent on a new footbridge with ramps and a lift for disabled passengers. You are proposing to make half the trains incapable of access by them.
It would be nowhere near half the trains, but yes I had overlooked that issue. When I was talking long-term about LHCS talking over Manchester - Swansea, that could be power-door stock. Right now though I'm more worried about getting the mark2 stock used to enable improved services in Wales in the 2014 - 2018/20 range. How old are the mark2s again, 2018/2020 is probably about right for retiring them. By then, we'll hopefully have enough DMUs, EMUs and more-modern LHCS (either new stock or power-door mark3s taken from Intercity 125s that are retired due to electrification) to run a network of usefully frequent services over all lines in Wales smoothly.