Remember the dichotomy is Free Will vs Nature. So the question becomes, "can you convince something (or someone) to choose to act against its nature?"
I think of that question in a different way:
"Can you change something's (or someone's) nature?"
Which is how I see Lash evolving. Her nature was changed from "fragment of a Fallen One" to "I'm my own me - maybe...". Maybe it was from multiple social takeouts that changed her high concept or maybe it was something else, but she appears to have changed.
And once she changed from "corrupt my host" to "protect my host" her new nature didn't allow any more freewill than her old one allowed.
Speaking of that... Spoilers for Ghost Story:
Dresden meets spirit like beings who have no freewill. They can talk to him, explain what they are going to do, but they can't decide not to do it. Harry not being about to talk one into doing or not doing something is one of the more frustrating parts of the story (at least frustrating for him).
Richard
Maybe we should define Free Will.
Let's talk about humans. A teenager who is the sixth generation of a military family, who has been to military school his whole life, could wake up one morning and decide to dropout. Head down to Burning Man and chill. Another teenager, one with hippy parents, could decide to ditch the peace and goodwill bullshit and join the army.
Neither choice is likely but both are possible. While people are the sum of their experiences they still have that spark. They can still wake up one morning and turn their lives around. To not just think outside the box but to jump from their current box to another one. To make an effort to become a radically different person.
Mab is Mab. She can do a lot of things but she can't dropout - she's tied too tightly in a web of obligations. What she does defines her almost as much as who she is. She's incapable of making certain decisions.
The average vampire is the average vampire. Joe the Black Court vampire can decide if he's going to stalk victim A or victim B, if he going to pursue this plot or that plot, but he has to stalk and he has to pursue plots. He can't just take off to Burning Man. His existance as a Black Court Vampire defines his choices as it defines him. An investiment banker could decide to give all his money to charity and join a Buddhist monistary but Joe is always going to be a Black Court Vampire. An aging hippy could take up a gun and start shooting everyone who doesn't believe in peace, love, and brotherhood but a Black Court Vampire can't to turn his (un)life around.
It's like having the choice of doors A to Z but you can't even think about using a window - or leaving those doors behind and dancing in the grass. All of those choices can give the illustion of freewill but only the illusion of freewill.
That said, the illusion is usually enough. Seriously, Harry doesn't say "Look what Mab's nature is making her do" but "What is Mab planning now?". She might not have freewill but she has enough choices available to her to fake it. Joe the Black Court Vampire doesn't have a world of options available to him, but since he can choose whether or not to stalk A or B, to fight or retreat, he has enough choices to fake freewill.
One of the best "freewill verse monster" descriptions in the DV is in the short story Even Hand (which is narrated by Marcone). To quote from that story:
I am a professional monster.
It sounds pretentious. After all, I’m not a flesh-devouring ghoul, hiding behind a human mask until it is time to gorge. I’m no vampire, to drain the blood or soul from my victim, no ogre, no demon, no cursed beast from the spirit world dwelling amid the unsuspecting sheep of humanity. I’m not even possessed of the mystic abilities of a mortal wizard.
But they will never be what I am. One and all, those beings were born to be what they are.
I made a choice.
And that's the issue of freewill in a nutshell. If Marcone wants to change he can. He could give all his money to charity and devote his remaining days working at homeless shelter. He could read to the blind or otherwise change his life for the better - he just has chosen not to. He is what he has chosen to be - but he still has the choice to change. Of course, he has enemies. If he did turn his back on who he is decide to become someone else then odds are he would be dead in days, but knowing that he could decide to reform.
Mab doesn't get a chance to reform. Joe the vampire doesn't get a chance to reform. Only Marcone gets to do that because he's the only one with freewill.
Richard
First: I need to find a copy of the book "Even Hand" is from and cannot do so. What is the title?
Second: Free Will is easy. All it requires is positive refresh. By that same token, if you let a player pick up the role of a fae or black court vampire...for game purposes (cannon notwithstanding) you just gave it free will. We can wax philosophical about it all day, but in the end...according to the system it is as easy as that.
I don't think that's a good answer as it would deny the vast, vast majority of humanity (i.e. all the NPCs) freewill and that's something that the setting doesn't support. For PCs the dividing line is positive refresh, but for NPCs there are "monsters" and beings with freewill.
McCoy can decide to duck his responsibilities as the Blackstaff to take care of something else, but Mab is Mab.
Now here's a spoiler for GS:
Harry encounters someone/something he can communicate with that doesn't have freewill. He tells it to do something other than what it's meant to do and the response is that is can't. Harry says it's easy, just decide to do X, and it says something like 'It's easy for a human to do that, but I lack freewill'.
It's not some kind of robot - it can make decisions, decide what is the best way of doing something, react to changes in its environment, talk and reason, but it can't say "take this job and shove it". It will intelligently complete its task but it can't decide if should complete its task.
Second, Even Hand was in the book Dark and Stormy Knights - a collection of stories about dark 'heroes'. Several of the stories (well, at least 2) were done from the points of view of secondary (and dark) characters from on going series. It's one of three stories (Even Hand, the story from Our World, and Curses) that wasn't collected in Side Jobs.
Richard
(who still has to track down Curses)