Agreed.
Yes I think there was a car train experiment some time back ... has to research it to know more though.
I think the lead changes due to the longer distances perhaps but it doesn't always of course ! Plus the more open scenery allows for better vision that in say Cumbria or Cornwall where there are greater hedges and less vision.
I agree totally to the sensible 'deal in one stop' when and where possible.
The overtaking does happen as it is common to allow those to pass, using the passing places !:lol:
I was given way to once by some cows that had learned to go into the passing places to allow the traffic (odd vehicle) to pass !
Certainly when I have had a horse on the road stopping once is always better than many times.
Slight aside ;
Horses are more 'settled' when in forward motion so are less spooked when busy than stopped. Keeping horses settled is important for safety. All horses no matter how well behaved can spook at something so by working hard to keep them settled is always sensible.

Much of this is true too when travelling them in horseboxes, and when travelling on smaller roads that are bumpy stopping to check them regularly is necessary, so going in a convoy is less favourable and hence why I'd let everyone pass and (then) keep my vehicle separate from others as much as possible.
Travelling close behind a horse box is never good practice as if your vehicle noise spooks the horse, and a horse panics (although most don't) if it chose to kick out, that box is no container that will hold it!