Ok, several things to somewhat answer here:

RE. Vehicle transfers
It's random, however very rarely has any cascades as a result of new vehicles resulted in the cascading of vehicles outside the North East region (the Scania Gas Buses in 2014 and South Shields' E200s last year being the most recent examples). Usually, stock is retained within the North East and reallocated where needed.

RE: E400MMCs
Unless things have changed, there used to be an X63 working that worked onto 62/63 on a evening. If that remains, I wouldn't be surprised if, for sakes of retaining the same standard of vehicle used in the 62/63, Walkergate's spare E400MMC isn't allocated to that board.

RE: E400s to North West
It is currently going aroind (from a well known and trusted poster on other forums) that Cumbria and North Lancashire are due to receive cascaded E400s to ultimately replace Olympians. Nothing at present suggests that any of the Newcastle examples will be heading that way however.