Dr. Whitehead wrote me back personally. He seems very personable from his email as a whole.

What he does in the perineum area.
"I'm happy to help answer some of these questions for you.
The bulbospongiousus muscles are completely dissected.
I do leave a very small amount of corpus left, which remains deep and buried, and gives a very small erectile sensation and heightens arousal and sensitivity during intercourse.
Cowpers glands are left in tact as they do contribute to neo-vaginal lubrication.
We do a scrotal graft and temporarily (the stitch comes out in a week or so) bolster to the anterior abdominal wall when creating the neovagina.
We find that this is sufficient in most transition surgeries."