Yeah, most of the guards now place 170 first class out of use in order to give themselves a safe space and also provide room for other train crew travelling pass etc. Most of the time the trains are so quiet that there's no real reason for any passengers to be in there anyway. Wait till you get an HST - some of them place the whole first class coach out of use!!
The reason you were asked to move at Invergowrie is a strange one, but I'll answer as best I can. 170s only JUST fit on the platform there, the cabs are on the ramps but the passenger doors are on the platform. So the guard in this case didn't want to despatch from the cab because of the large step presumably, and wanted to use the saloon door. To maintain social distancing, they have asked you to move so that they can do that. Seems ridiculous to me, but there is absolutely no operational reason to do that, it must have been their own choice for social distancing.
In answer to the other points, TE's are allowed to be in cabs as its the only secure area. However, unlike guards, they are expected to have a valid reason to be in there - i.e. the train was too busy to be in the saloon, or they felt threatened type thing. They get a small degree of traction training regarding where the ramps are stored and how to access the train if its locked up, that type of thing. They can't operate doors, they don't have a PTS certificate, they know nothing about signalling, they are effectively just the same as a passenger in an emergency and they have no minimum knowledge whatsoever of the routes they are on. They are not safety critical and don't need to meet the medical standards for that - that is to say they can be colour blind, hearing impaired, epileptic etc. I'm not belittling them, its a valid job, but they have one single duty and its revenue collection.
Oooft, only mainline drivers can drive a train on the mainline in ANY circumstances, even moving it a millimetre. To give you an example of how touchy a subject that is - the Driver Team Managers at most depots will not even start a train up from cold without a driver being present with them. In theory DTMs are allowed to drive a train in dire circumstances, to clear the line following an incident etc, but they can't go out and drive trains in place of a driver...even though they assess drivers all day and most of them have a lot of experience as drivers themselves. The reason for this, which sounds bizarre I know, is because they don't drive frequently enough to maintain the level of route knowledge that a normal driver has. Also because...ASLEF