Not necessarily. Here is the definition of "lie" I found:
If the seven words were using the second definition of "lie" then Uriel is in the clear to say them. We have a WoJ that Mab could say that she could change Harry because she believed it to be the case, given her past experience with Winter Knights. Thus, Uriel was just saying that her claim was founded on a mistaken impression, which indeed it was.
The difference between a lie and a mere mistaken impression or falsehood is in the knowledge or intent. In the example you used, the married woman would be "living a lie" if, for example, she was pretending to love her husband when she knew that her feelings weren't real. Or maybe her spouse was a criminal and she was pretending her spouse was an upstanding citizen. Either way, it is about willful blindness. I don't think the Fae can do that - their subconscious would force them to confront the truth by preventing them from lying. Mab would have been physically unable to speak those words if she was merely hoping to live a lie by lying to herself.
Why wouldn't Uriel be able to lie? He's the spook, I think he CAN lie, just not in a way that compromises Free Will.
I think DonBugen handled this question quite well, but even assuming Uriel can bear false witness on some occasions, lying to somebody when you are telling them something it is extremely important for them to believe is not very smart. E.g., 2 + 2 = 3, trust me, Mab can't change who you are.
More importantly, he says lies. He doesn't say 'Mab lies' he says 'Mab's words are lies'. A semantic difference, but if Mab is wrong and doesn't know it, you could argue her words are still lies withou Mab lying.
Mab lied, Mab's words were lies, and Mab told a lie are all synonymous. The fact that you felt the need to specify that "you could argue" is a clear indicator that "semantics" is a poor justification for using the word Lies. Either JB made a poor word choice by using "Lies" over the equally available and more accurate choice of "Wrong," or JB chose the word lies because Uriel and was not the actual speaker and the word choice was tailored to the character speaking. JB isn't perfect, it's possible he just made a writing mistake, but in this case I don't think he did.
We should also not forget, that when Uriel in Ghost Story reveals the seven words whispered to him by the Fallen, that scene is different and that thought does not actually occur in the text during Changes. Even though we learn his memory was altered by Molly, that scene does not actually exist in the text of Changes.
Agreed, but why is that important other than that JB skipped the scene in Changes for story-telling purposes?
And I do agree, just because Uriel called Mab a liar doesn't mean Mab though she was lying. She simply assumed what she said was truth. I don't think the fae have to tell objective truths, only believe that they do. Otherwise the fae would be WAY more powerful than they are. I think the only beings that might fall into that category are beings with Intellectus (funnily enough, like Archangels and the Mothers).
If Mab didn't think she was lying, then Mab wasn't lying, she was simply wrong. Telling a lie requires knowledge of the falsity of the statement.
Mab's lie is a lie of omission.
That just doesn't fit. I don't think Mab was being clever and trying to mislead Harry into believing something that wasn't true, I think Mab believed what she said was true and she was simply wrong, because she doesn't truly understand free will.