Good questions.
There was a deliberate policy decision years ago that it shouldn't be possible to excess either the outward or the return portion of a return ticket from off-peak to Anytime (using the terminology we have now, rather than then) for half the difference in fares. Instead, the full difference should be charged. But, I think you may be right that if you're doing a change of route excess, there's nothing in the internal guidance to suggest you can't at the same time change between Anytime/off-peak/super off-peak and still benefit from being able to pay half the difference.
As for what happens if you can't find somebody to sell you the excess, that's far from clear. There's no longer a right to a change of route excess set out in the NRCoT.
I'm looking at the definition of "Ticket" in Annex B NRCoT. It neither expressly includes nor excludes the notion that, with respect to your intended journey, you may require two or more tickets (though that is expressly permitted elsewhere). An excess is, of course, not valid by itself, but that is also true of some other tickets (e.g. Boundary Zone tickets). That doesn't mean it's not a ticket.
So I would say you could argue, if you held a ticket that wasn't valid for the route you wanted to take, that while you are now required to get a change of route excess before commencing travel (last paragraph of Condition 9.5), you are then in a position where you require a particular ticket (namely an excess) in order to fulfil the requirement at 6.1 to hold a valid ticket, before you board, and the exceptions at 6.1.1 apply.