A rescue loco couldn't sit at Chester. There is simply no room in the platforms or even parcels, and no Chester crew sign it to keep it in the nearly constantly used yard.
No other depot signs Chester yard, or even the shunts. Same for Abbey Foregate, Salop crew only.
IF and that's a big IF they decide to use more of these, then the most sensible option would be to train a small amount of Chester drivers on them. It would then have proper resilience covering all routes across the Coast, Crewe, and Salop. You realistically could have two drivers (Cardiff/Chester) covering the entire route. It could even have specials running from Liverpool through to Cardiff on sporting events.
Salop and Crewe drivers don't cover the coast, Junction and Holyhead don't cover Crewe-Salop and most don't cover Chester/Salop
The traction learning cost would be prohibitive, but it would make the services much more reliable. It's not going to happen because of the cost, but it would be the most beneficial.
I imagine that's correct RE: Chester although there would be the other cost obstacles anyway that would make sticking a thunderbird there unlikely. If it wasn't the case then either training would have to be undertaken on the sidings or you are reliant on the non passenger bay near the entrance at the Crewe end being free as that's part of the station infrastructure and units go in overnight. Equally likely you'd have a Crewe or Shrewsbury option anyway if you were having a thunderbird in the central parts of where a train might ever need a rescue. As said in my other post I suspect cost would prevent it. Having a second spare loco at Holyhead rather than just the Canton one would seem more likely.