This is about as complicated as it can get.
First of all, AIB were under no obligation to allow your ex off the mortgage. They did so because you were able to pay off a lump of cash and then you could afford the remaining mortgage on your own.
Likewise, they were under no obligation to offer you anything other than a standard mortgage to replace the old one.
There really is no basis for AIB restoring the tracker to people in situations like this. Fair play to them for doing so. I doubt if the other lenders will be doing so.
When a couple split up and want to move their tracker to new properties, AIB allows each of them to move half the amount.
So that seems to be the approach that they have taken here.
Has your ex. used "his half" of the tracker? Does he intend to do so? If so, then I don't think that AIB should have to provide you both with tracker mortgages.
If your ex. waives his "claim" on a transfer, then I think it would be logical, although very generous, for AIB to give you the full tracker at the original rate.
Has AIB sent you their redress letter yet?
Brendan