I think that I'd scrap the foam curb and make a new one. Note, though, that it is possible to cut the curb in both height and width (width would need to come out of the middle as a slice, though). Cutting nice and straight is easy with a table saw, but can be done with a circular saw and a straightedge. You want the curb to be at least 3" above the pan. If you slice a section out of the curb, just put some thinset on that joint when setting it. Between that and the thinset on the floor plus the Kerdi fabric, it will be fine. You could use Kerdifix to hold the thing together, but since it's going to be covered with the membrane, that's kind of overkill, but the KerdiFix is a REALLY strong adhesive.