In theory an "only" dosing claim can be made. The specific claim and/or copy required would be assessed during the review based on the indication and limitations of the "only" feature.
Yes, a disclaimer is required when making a non-clinical claim. In this example the copy is inherently comparative against the dosing of all other products. The disclaimer would be required to read something similar to “Comparative clinical significance unknown”.
The extent of the dosing copy will be dependent on the dosing in the TMA, the overall message of the piece and any class features which may impact the interpretation of the claim. In general, you will also require copy reflective of “for full dosing and administration, please see the Product Monograph”.
A letter from medical/regulatory confirming the nature of the “first” and/or “only” claim. The sponsor should also recognize in that letter, that they are aware that it is their responsibility to pull all pieces with this claim from the market if it should no longer be true, regardless of time left in the approval window.