There is a third option: temporary forms of an intervening temporary entity--whatever it was (neither Dominion nor Republic). This makes the least sense practically, because it's arguably needlessly expensive to make and stock forms that will only be used for one year. And what would the intervening entity be called, anyway?
Logically and practically, it makes most sense for Kenya bureaucrats to have been using the new Republic of Kenya forms in February 1964 inasmuch as the Dominion was dead and gone and the Republic was a "done deal" just waiting to be fully implemented.
Sure, I agree. But that is a matter which can be readily authenticated.
All we can do is speculate...until such authentication is actually conducted.