Isn't there an issue with consent with the third law as well?
The Merlin example, he could assume consent for directing how to handle the dangerous situation, being the Merlin.
You can't assume permission. That has to be given.
However, the Third Law isn't "never send people telepathic messages" or "never use telepathy in any form" it is "Never INVADE the THOUGHTS of another." Sending a message is not poking around in their head. We see it twice in the books, once between two wizards sending messages back and forth, and once one-way (by the Merlin). If you aren't looking at their thoughts in some way, then you aren't invading their mind, imho.
Molly invade Luccio, apparently, when she just looked around to make sure nothing was screwed up, I can see how that would require poking at thoughts and such even if you don't get anything specific on what those thoughts are.
You do bring up an interesting point though. If someone gives you permission to check out their head, that seems like an invitation to me, and hence that person wouldn't be invading (unless the violate the mind in some fashion while they are there...like a really bad houseguest). The books don't really go over that, but they do go over how people get their minds messed with and are being treated for it. Seems like there'd be some element of permitted poking around to assess the damage and find a good way to fix that up.