William does not give me the vibe of ESTp in any way, shape, or form. Merely showing a photo where he is seen at a particular angle means about zero. Looking at him here:
YouTube - Prince William And Kate's First Interview Since Getting Engaged
his eyes, in terms of how they move, do not show any Se qualities. He entirely lacks the kind of attentive physical presence and immediacy I associate with Se leading. On the contrary, he seems more Si-valuing. That is, his frame
could be EJ (not sure yet), but his presence is soft. I will say re temperament, he doesn't have that might-go-all-over-the-place quality I see with EP temperament.
And in that video, although of course he is asked some mundane questions, his answers are just strictly mundane to me. That is, perceiving him through my own bias for Ni, I find everything he says, with its focus on the concrete operational details of proposing marriage, about as interesting as the description of a brick. At least some Ni-valuers, if you ask them a boring question, will begin offering feedback and ideas that are more interpretive or philosophical. He's quite content to delve into a narrative of "then ... then ... then ... then ..."
My first thought for him would be Delta. And there's a moment in that video when Kate seems very infantile to me. But not sure at this point.