In theory there would be ways to "pact" with a fairy lord for magic - and I'm basing that off of the other forms of sponsored magic. That said, I can see a mundane having to perform duties in exchange for power. Life altering duties.
A while ago I posted a Knight/Emissary of Santa Claus (at
http://www.jimbutcheronline.com/bb/index.php/topic,23084.msg991664.html#msg991664 where someone's life was completely altered when he agreed to be an emissary of Santa Claus. He ended up with a variant of unseelie magic but his new office dominants his life. That's the sort of price I see a normal mortal having to pay for fairy magic.
As for changelings, are they "human" enough to be lawbreakers? I think they are, at least until they make their choice to be true fae, but I can't make a strong argument over that point.
My view is that "lawbreaker/not lawbreaker" all comes down to "does the sponsor do the magic" or "does the mortal wield a small part of the sponsored power"? If the former then the mortal isn't working black magic - just serving as a vessel for it - and is no more a lawbreaker than someone using a magic belt.
If the latter, then we're back at why lawbreaker exists. Repeatedly throughout the books we are told that a wizard can't do something magically that the wizard doesn't believe in. That a wizard can't kill someone with magic unless that wizard believes that murder is all right - and in the game that mindset is reflected by the lawbreaker power.
Richard