I don't think he can turn it on or off, consciously or unconsciously.  It's a force field around him, or a rose, or a ring, all the time.  The only thing that changes is the hunger in the Wamp, which is on or off.  If the hunger is off, then contact can be made, like between him and Thomas, or him and Lara in White Night.  Only if the Wamp's hunger is always on, like Madeline's in Turn Coat, is the protection a threat to the Wamp.

This suggests that Lara is either walking around always on, with no control over her hunger, or she's so attracted to Harry that she can't help but be attracted to him, and the hunger feels it too.

The White Council officially knows about the island and ley line.  The Gray Council knows about Bob and Amoracchius from Changes.  Seems likely that at least Vadderung knows about the items from Hades vault, given his habit of knowing things.  Someone recognized Margaret's necklace in BG (can't recall who at the moment).  The Eye of Balor is highly suspected.  Everyone saw the Little Folk answer his call.

I'd say it should be very obvious to everyone that he's OP.
Poor Carlos.  If only he'd been a little more understanding, Harry might have let him take a nap with the Shroud for a bit.
I think we've got Mirror Mirror, the wrestling book, and maybe two to three more casefiles before the BAT.  Should be interesting to see where things end up, but I'm wondering how long it'll be before he ends up becoming the Baron of Chicago himself.  I figured Marcone would last until the BAT, playing the Uther to Harry's Merlin, but I'm not so sure now.