Hi, Chris, and welcome to the forum.
- Why was the missing arrival/departure not shown at Totnes as "cancelled" rather than just removed from the display
There are some quirks in the systems sometimes ... I suspect that as the train was going through (but none-stop) it wasn't really cancelled
- Why are different FGW▸ staff giving out different reasons for the non-stopping, none of which is either comprehensive or entirely true
Very possibly because they didn't have the complete information themselves ... in this sort of situation there are usually fairly standard plans in place as to what happens, but it may not be know whether the engine's completely or partly out, and it could well be that they were reading the evidence they had to the best of their ability, giving you the most probable cause.
Information systems are far better than they ever were, but complete, instant and certain information when something starts to go adrift is still not with us.
- If Dainton bank is as steep as Rattery Bank why would a train which can't stop at Totnes be able to stop at Newton Abbot and still get over the top?
I would expect that a 125 on one engine could get over either ... but very slowly and with big time loss. I can't see FGW committing a train to go over the top on one engine and keeping their fingers crossed that it didn't get stopped at a signal because it might be stuck. But there's a big difference between 20 mph and 60 mph ...
There's also a big difference between Newton Abbott and Totnes ... and whilst there was room on the crosscountry service for Totnes passengers, would that have been the case for people going to Newton Abbott too?