Are any Mk IV + 91s intended to work on the Newcastle/Edinburgh runs at all?
At present the only services confirmed to be getting 91s from Tuesday are those I listed. It is possible for them to work to Newcastle (but not Edinburgh without route conductors), though I'm not presently aware of any plans for them to do so. Four of the sets do however need turning via the High Level Bridge, not clear at the moment when that'll be done and if it'll be done in service or not.
I imagine turning them the right way around isn't a top priority! I believe it's only only a one day prep at NL before they can go back into traffic.
It depends on a number of issues, it is preferred to have the loco on the north end of the set as it makes it much easier to swap the loco quickly if needed, and there's also the issue of last minute dashes down platforms when the train shows up the wrong way round - not too much of a problem just for 1 train but when 4 of them are running like that it's a bit more of an issue.
Isn't there now an issue with crew competence at Newcastle and Edinburgh? Better to keep them on Leeds/York trains where the crews know them and they can be worked back to Neville Hill for maintenance. Hitachi aren't interested in looking after them, hence the move to Neville Hill.
London, Leeds and Doncaster crew sign to Newcastle so the 91s could work to Newcastle without issues. Edinburgh would require a route conductor.
The vast majority of the current diagrams because of the Kings Cross works visit both Edinburgh and Leeds, so without reworking the diagrams (which may or may not be possible) it'll likely be difficult to keep them confined solely to the Leeds route if they end up with more than one set in service at a time.