Normally Kubrick wouldn't allow ad-libs, but he made an exception for Sellers. Very wise.

The Pink Panther was released around the same time as Dr Strangelove (also in January 1964). I'm guessing they were made concurrently. So yes, I don't see how it would have been possible for Kubrick to have seen a finished film (perhaps he saw some early rushes).

Anyway, thank you. Some fascinating additional detail there. :)