Kirk moving his head, and later having a conversation, is merely a good way to make explicit what production realities imply anyway. It's impossible for the actors to assume the exact same pose at both ends of the transporter process (unless it's the same set, in assorted "mirror universe" or "malevolent illusion" adventures), so we have to accept that movement "within the beam" is possible.
And the more coherence in there, the better for the machine. If the blob being pushed through walls and space retains more of it, the machine needs less data or computing power to put Humpty Dumpty together again. And while being beamed has been defined as oblivion of some sort (say, Chekov in "Day of the Dove"), it hasn't been defined as temporary death or total immobility as such. If it were that, Scotty could wait forever in the beam in "Relics" rather than risk a "cycle" that might incrementally age him till he died of starvation, and the hinted degradation would make less sense in a static situation than in a dynamic one, too.
But that's just the technobabble side. Studio reality calls for the ability to lift a leg when beaming to an uneven surface, and going with that all the way is a win-win approach.
Timo Saloniemi