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The possessive form of the singular noun notary public is notary public's.
example: This form requires a notary public's signature.
A law enforcement officer is a notary public.
I do not believe that there is an accepted abbreviation for the designation of "Notary Public."
If that accountant is also a notary public, then yes.
Only a notary public can sign. Often Lawyers are also Notary Publics as well.