This isn’t how it’s going to work. If this was to happen then you’d just keep the peaks and then if something breaks on a core service then the peak would swap, this already happened often, requires permissive working (two trains in one section) with the drivers switching units, but this was already done when needed.
Removing the peaks means those trains will be in the depot, sat around, if you require a unit swap then a driver needs to get to the depot, prep the train, then get it out of the depot and back in service.
To put it simply it’s a total pooshow, Nexus haven’t a clue really how to deal with this and as always buried their heads until the issues become so large they’re no longer possible to ignore.
I think the permissive working your describing can(or at least have) happen at anytime of day as long as their is a spare driver at the depot so in theory the removal of the peaks should not affect that.
For me though, it's the mixed messaging, you hear words like "permanently" but then the MD comes out with they may return when demand provides it although the cynic in me really do think they won't return even if fleet availability improves and it seems to be an understanding they won't return even when the new fleet is fully operational. I'm assuming that is because the frequency will marginally improve(which was one of the reasons why the Shields line was duelled) by then and the more open space means more people should be able to board more easily.
Going back to the new fleet, seeing all the photos of the interior of the new train, I'm sort of warming to the new design layout, the merseyrail set seats have been complained about quite a fair bit despite being in a position I would prefer and despite on the photos look alright enough goes to show photos can be deceiving. I do think despite the more standing room, I do question when the train is busy what on earth can passengers hold onto if your in the middle of the crowded trains and the handle bars are too far away to reach, i think I read hanging straps from the ceiling can't be added because of safety reasons(?) so it's going to be interesting too see what this will be like in practice.
Also one thing to add, I'm noticing the wording for when the new fleet could be in passenger service is changing from "the autumn" to "towards the end of the year", I think we can all make out what that means. I'll be surprised if any make it into passenger service this year.