Can the doctors/nurses access the cranial nerves or other major nerves through the incisive canal/incisive foramen or any other opening in the palate?

No. The incisively canal is an exit point for the palatial nerve which is a terminal branch of cranial nerve v. It isnt an entry point into the brain or cranial cavity.
Branches of..... The trigeminal or 5th cranial nerve, is the nerve responsible for sensation in the face and certain motor functions such as biting and chewing. Its name derives from the fact that each trigeminal nerve, has three major branches: the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. Injections of the palatine nerve, or others, are just small end branches of that cranial nerve.