This caused the next unit in the queue to be renamed a very odd name, something 'King Roman Numerals #' - removing him and then putting him back caused a CTD.
This would have been because you were retraining the heir to the throne the year the king died. I have encountered this problem only once, and it corrupted a great campaign I was having In my case the problem was down to me "helping out" another faction, by switching factions and sorting out its training queue. Of course, I'd queued the heir, the king died, then there were two kings - one out in the field, one stuck in the training queue with a unit size of 255 - MTW can't handle units over 200, so it CTDed if I ended the year, and CTDed if I tried to cancel him from the queue. Game over, as the previous save was about fifty years earlier

There is one other bug I've come across, and this one is down to not paying attention to detail. It is sometimes possible (can't remember exactly how) to open a training parchment to one province, whilst looking at a unit in a different province. This unit will load into the parchment, but because it isn't in the "right" province, something nasty will happen. I wonder if that's what happened with your Highland Clansmen? Maybe you had the Northumbria parchment open, but were able to pick up the clansmen anyway? It is a very strange one, I admit, but it can happen.

Usually repatriated troops become selectable the next year, sometimes it takes longer, but Ironside has the answer - merge stacks. That will free up the blockage.