If you look at a DB Regio DD you'll see it's effectively built as a multiple unit containing driving cabs but with a loco at one end so there's no need for the loco to be switched to the other end or for a DVT to be used.
That is not a MU, it is a standalone loco hauling unpowered coaches with Push/pull capability.
I am not saying that this arrangement is bad but it would be an about turn on British railway policy since the 1980s of abandoning loco hauled passenger trains for MUs.
There is also the small issue of no current design of desel loco available for the GB market that is suitable for passenger use. The costs of a new design or adaptation of a continental design for a small niche fleet would be too high.
However, as Hazel Grove to Manchester is electrified they could be dual-fuelled or the OHE could be extended.
How would any of this be a better solution to the relatively straightforward solution of some platform lengthenings? In a fantasy situation where you had infinite resources to play with maybe but in reality there is no chance of an outlandishly expensive solution being chosen over a duller but more cost-effective one.
Very high capacity electrified routes that have already reached the limits of traditional capacity enhancements are the only places where extreme solutions such as altering the loading guage to fit double-deckers are ever going to be seriously considered.
I think even in the latter, an emergency back-up diesel engine would be needed on the Buxton line, you don't want a power cut to be followed by a train rapidly running downhill - which is what they thought would happen if Pacers were on the line and an engine were to cut out.
Why would a failed engine on a pacer (or a power cut on an electric line) lead to it running down a hill out of control? That is what brakes and their associated failsafe systems are designed to deal with. If that were a realistic scenario pacers and electric trains would be a complete danger anywhere with gradients, they are not.
The restriction on the Buxton line is there because certain low powered units would be unable to maintain the running times under normal circumstances and may be unable to climb some of the gradients in the case of a partial failure. A pacer on one engine may well stall but it wouldn't suddenly roll away out of control.