I don't think anything other than E400H's would move to Metroline, the E400H's might pass because most of them are based at Manchester by Stagecoach or are now owned by TfGM anyway, and they are also very troublesum, Nothing else would pass to Stagecoach, especially when Stagecoach group have a large number of ALX400's and E400's that could be replaced overall throughout the Group, and of course loans at its Bee Network garages. Although there is nothing to say that Metroline might loan older E400's and then return them to Stagecoach as replacements are sourced, but any E400MMC would be wisked away straight away.
I think you may shortly see a large double-deck order from Comfort Delgro for Manchester quite soon, plus the transfer of their own London fleet, and also purchases of Secondhand Euro 6 ex London Buses from various dealers, then there are odds and ends in Wales, and who's to say they couldn't visit Waarrington and purchase their newer deckers e.g. the two E400 City's plus the other E400's and maybe the ex-Isle of Man B9TL's when Warrington go all electric later on this year. Then another source maybe coach seated E400MMC's from coach operators who may return them off lease, or decide they need to sell them because they aren't making enough money from them or lose school contracts in the Summer.