Nope, I'm just not seeing that way.  Sorry.  We'll have to differ.  I don't see it being anything like taking sides.  He's putting up a passive defensive measure for his patrons.  I don't think that phases him at all.  I interpret it as him just not liking Harry knowing, and having it be learned by others. 

If anything I could see him not wanting to get involved in big things for other reasons, and wanting to be 'out' of it, but I think that's more about remaining anonymity, which he's losing by Harry learning about him.

I don't really understand that Mac was supposed to ask for help. I see it more banal, if he fells the bar fells.
What do you mean? That any attack will be redirected to him or that the attackers would actively search for him? Because I think it's the latter (and in fact, some attackers may not know how the protection is working). And then, Mac may be clever in choosing to hide (or better, blur in the background) better than actively tanking.
Also, I disagree with your interpretation about his unease. He specifically sighted before smearing his blood. I don't know exactly why. Perhaps it is because, as Mira says, he was choosing a side. Or perhaps is because he had too much devotion for the placard, perhaps he didn't feel worthy of doing that. I am not sure.
And I still don't know who told Harry all that about the pieces but I want to know what all of them (including the Grial) do in the Dresdenverse.

I guess we're just interpreting it differently, like Mira said.  I don't know, I took it as the placard would put up a barrier to keep things out that were attacking.  It's not powered by Mac, just as the other vault items don't seem to be powered by the wielder. 

But Harry clearly states that it'll be dependent on Mac, that it "focuses energy on an individual", and that "Anything that wants to get to them will have to go through Mac first".

To me that sounds like it's creating a threshold that links to Mac.  So either he dies if it falls, or the energy is directed at him, as described by Harry. Either way it's a lot.

But as for his consternation, to me it's clear that he's not nervous about the blood on the placard, but instead is nervous about Harry figuring out what he is.  Harry even thinks "it was pretty clear that he didn’t like that I’d realized what he knew."

Not that he was nervous about the placard or choosing sides or risking himself, but just Harry knowing that he was more than human.

No, it isn't, but that isn't something he wouldn't have done anyway, he didn't need to be asked or to add his blood to seal the deal.  As Harry pointed out, Mac knew what the Placard was, what putting his blood on it meant.. 

Mac knew what was going to be asked of him, not the same at all as looking after the people in the bar. Mac would have done that anyway,as best he could. The Placard is about more than just that.  Mac himmmed and hawaad, looking at the sign, then away..

Choosing to put his blood on the Placard, if it didn't cross the line for Mac, it blurred it to the point where it might no longer be there..  Because;

I looked up the meaning of "intercession."  According to Wikipedia;

That my friends is taking sides, it is a pledge from Mac, that he'd be willing to ask for divine help to save those in his bar.  The guys he refused to back many millennia ago in another battle between good and evil.

That's not my interpretation of the scene. My interpretation is that the plaque paints a target on the person who's blood is on it. The protection offered by the plaque is connected to Mac, so any attack would come at him. 

That's a huge difference from helping defend. If the plaque had been present in the Cold Days attack, then rather than Before breaking the threshold of the place and attacking all three there, he would have had to have gone through Mac and killed him.

It's putting a huge, inescapable target on his back.  It makes a huge difference in regards to strategy.  It makes him the Tank. And if he isn't strong enough, he'll wind up dead.

And as for his reaction, his unease wasn't just putting his blood on it, if it was that at all. It was that Harry had deduced what he was. 

Yeah, I wasn't making a case for or against any of the theories.  I was just adding depth to the WoJ, which likely stemmed from that convo or similar ones elsewhere back in the day.

I'm fine with the idea that Margaret was only 40.  I'm fine with her being 140 but looking and reproducing like 40 because of good genes (obviously from her mother's side) and magic (anyone doubting that Lea would give a little help to a friend in exchange for something equal?) and varying magical biology (Mab was mortal once but still had kids after a few hundred years or so, so there are ways).

My specific question to Griff is whether or not black magic corruption is Outsider corruption. I ask this because I believe Harry was corrupted by black magic. If a starborn is immune to Outsider corruption, then Harry would be immune from black magic corruption if it was Outsider corruption.

Harry isn't immune from black magic corruption, therefore black magic corruption isn't Outsider corruption. (At least that's my thinking assuming starborn are immune to Outsider corruption).

Now it could be that starborn have really strong resistance, not immunity, to Outsider corruption. This would explain why Harry didn't go full warlock from his corruption and why borderline 1st Law violations don't seem to have any affect on him. Killing the Turtlenecks with magic might make a regular wizard go full on cackling evil. But for Harry, it's no big deal. (Or they're not 1st Law violations at all). I'd have to carefully parse what Eb said in PT about it.

I think magic a spectrum of energy, with different types of magic working at different frequencies. 

I think Outsider magic (spells, incantations) is on a specific frequency that goes Outside.  I think the Outsider frequency is in the dark end of the spectrum, so it's dark magic, but not the same as other dark magics.  Hellfire is a dark magic, but it's a different frequency. 

Likewise, faith magic, soulfire, and other examples that we don't see much of are light magic.

I think mortal magic is unique because it's basically neutral, but the intent and the result can both change the frequency.  If Harry casts fire to light a candle, it's not dark.  If he does it to kill someone, or he inadvertently kills someone with it, then a spiritual backlash occurs that taints it as dark.  That pain and suffering that his magic causes splashes back on his magic/aura/spirit/soul, and it stains it.

It's easier to picture it as paint.  If every mortal starts off with a gray base paint, then how they use their magic will affect the color.  Do something dark, like killing someone, and you're going to get a splash of black in that gray.  Use your power for good, like making a baby laugh, and you're going to get a splash of white.  And since every type of black magic is its own color/frequency, it can be distinguished from others.

Likewise, if you come into contact with magic that's dark enough, and it comes into contact with your spirit/paintcan, then stains can be left even without your involvement.

Isn't there a WoJ that she was very old when she gave birth to Harry and was only able to conceive at that age because "wizards cheat."

That was due to Ms Duck and others arguing that JB didn't take fertility into consideration regarding her age.  The argument was that if she were old enough to be the daughter of Eb and his wife from over a century ago, she wouldn't have been fertile.

The possible solutions were that she aged slower due to a lot of time in the never-never (thus accounting for her fairly youthful appearance in the Raith painting and Harry's vision), or it was 'magic'.

Saying she was forty when Harry was born is likely on the very low end of her actual age.  She was most likely over a hundred, but somehow looked considerably younger than other wizards do at that age, and was also still possessing viable eggs.

manipulation sure, but I'd argue most of those were still him making the choices on some level. The WK mantle makes his inner voices louder, Lash had subliminal discussions with his ID, he ate the nightmares chi, Evil Bob mesmerized him, but in the end had to try old fashioned recruitment too, ect. Though of course he's not immune to manipulation, no. Like the WCV's power. Or what evil Bob did. But most of these seem to work on other levels than purely psychic, which seems to be the ultimate display of power, Willpower.  I think that's where he's got the edge. Even his display against MW was the will of one tiny mortal against a cosmic level being. Course... She might be kinda outsiderish too...

It's a slope, and what we've seen is him at the very top, putting one foot out. It starts with very subtle shifts in personality or behavior that he doesn't notice. Then it gets worse, and worse, until he's acting out of character for what he originally was.

Until...

I think the mantle is doing more than making his inner voice louder.

Exactly thia. He spent basically no time fighting the mantle in Battle Ground. He leaned into it a few times. He recognized it influencing him in the final battle with Ethniu, and recognizing her vulnerability, and it didn't caution him.  He felt it leading his bannermen, and it didn't phase him. He embraced it on the march though the city.

He's fully capable of going dark side. He's just immune to the specific threats of the Outsiders.