Your issue will be around being able to teach towards an IR without holding a CRI as it will be the ops manual that governs the qualifications required. Our ops manual lists the qualification that must be held on order to teach for each of the courses that we offer.
This was included at the request of the CAS when I did the EASA compliant manuals and was the reason that the whole discussion with Timothy kicked off. My ops manuals requires an IRI to hold a minimum of a CRI and according to my flight ops inspector that all the ATOs will have to observe the new regulation.
This means that even if you win your battle and get some sort of grandfather right you will only be able to teach towards the IMC if that is acceptable to the CAA.