Very valid points there.
My 'Fiddleyard' would (most likely) be up four cassettes stowed underneath the layout, each would span over a metre. In terms of times in between trains, for off peak services, the gap between trains would be 1 - 2 hours. However since my layout is a dual platform terminus, the trains would remain at the platform for 5 - 10 minutes, at times up to 15 minutes if the service they run goes back to the depot. Somewhere in between that waiting time another train would run into the second platform. This would give the 'All Clear' for the first train, allowing it to depart. I'd run departmental and parcels trains in between too. These would come once or twice a day. Peak hour times would be halved, meaning that trains would be only 30 minutes to an hour apart. And the trains would leave a bit quicker too, meaning they only wait a few minutes before departing.
Getting to York from the junction station would take about half an hour, assuming that's how far away from York it is. If I was to run only one DMU and one service, it's pointless to have a dual platformed terminus, and really there's a lack of variety. I'd prefer to have a few services or a few different trains to add variety.
Regarding the reason why the trains are mostly DMU's, this is what a branch line would usually have. It's only really the 1980's period that has more DMU's on my layout, as I'd think BR would have eliminated most locomotive hauled services from branch lines. It's also a pain (for BR) to operate them on my terminus as there's no run around point, meaning the locomotive is trapped. Another locomotive would have to come along and couple up, and depart with the train. The other locomotive would have to go light engine and wait in a siding for the next locomotive hauled service, and then take it when it's ready to depart. After that it goes in a cycle, over and over again. This is the sort of thing I'd do with the parcels train, and a couple of the 1960's services. There is actually only one DMU service in the 1960's, the other three (including the parcels train) are steam hauled.
Multiple working is also a bit of a pain for BR. My station can only fit a three car train at the most. 4 car sets would have to comprise of two 2 car sets. These would split after they arrive and the rear half of the train would have to go the other platform. If necessary, at departure they would couple up again and leave as the 4 car set again.
Shorter two car trains would provide shuttles along the branch line, whilst three or four car trains would continue to York. In the 1960's as you say locomotive hauled trains run the through trains, whilst the single DMU unit will do shuttles to the main line.