The old LMS standard gangways were part of reducing the weight of the original alloy-constructed Derby Lightweight units, along with the short 57' length - typical minimalist LMS approach that still pervaded Derby. Notably the Western Region would have no truck with 57' cars, lightweight bodies or bus seating. It was also felt that few passengers would need to pass between cars. Remember on local services they were usually replacing completely non-corridor compartment stock. The gangway approach then carried forward to all the remaining short frame cars from the independent manufacturers, for interchangeability.
For why the form of transmission, you only have to look at the waste of space on a Southern Region 2-car unit, where virtually half of the body of one car is taken up by the engine-generator set, a huge radiator, etc, and a noise that made people put their fingers in their ears when multi-unit lashups passed. The dmu premise was that the power train was straight from commercial vehicles, both engine and gearbox, all underfloor. The bus industry in the 1950s had just moved from conventional front engine mounting to horizontal underfloor engines, and all the lubrication etc issues with engines mounted "sideways" were addressed. It has a nice weight balance in the car with two engines and transmissions, mirror imaged. The difficulties with the later Cravens cars which only had one larger engine, all cars powered, but unbalanced for weight between the two bogies and significant vibration issues, a classic "it seemed like a good idea at the time", caused their early withdrawal. The railway bus/truck approach came from the pre-war GWR cars, by AEC/Park Royal, and an equivalent LMS set with Leyland engines, which formed prototypes.