Maeve was at Splattercon!!! disguised as a vampire.

[Elegast]

Thanks to Griffyn612, we even know the reason for each attack.

The first attack was targeting Pell. Aim: free the theater,where there is a Way to Artis Tor.
The second attack was to make Harry react, and make sure he would send the phages to Molly.
The third attack was targeting the vampire/girl to make sure there would be no witness.

The third victim had been one of the little vampire girls I’d seen the previous evening. I could only tell because her head had landed facing me. The rest of her was hopelessly intermixed with the other two bodies.

Time resumed its course. The energy that powered the spell fled out of me in another rush, and left me lying on my side, struggling to draw in enough breath. I could feel the spell sizzling down the lines of power for the summoner, and a heartbeat later there was a sense of impact as the spell went home. As it happened, the entities my web touched went abruptly still, the web ceasing its trembling—and then they all surged forward into sudden motion, vanishing from the web, and presumably streaking after the lure.

All but one.

That's why Harry's spell failed. Mab knew Harry was planning a redirection spell. The last phage was specifically send to kill the vampire/girl and could not be diverted.
The fourth attack was targeting Glau, killing the last witness, saving Harry if necessary, and taunting him.

[Arjan]

Elaine warns Harry against taking Maeve's apparent personality seriously, in SK, though she's in the process of deceiving him and trying to keep his suspicions pointed away from Aurora at the time.

But Maeve's behaviour in Proven Guilty actually supports this. Maeve was acting in Summer Knight and Harry fell for it.

And Fix should know:

Fix sipped at his lemonade and said, “For the love of God, Maeve. Would you
give the Evil Kinkstress act a rest? It gets tired pretty fast.”

She is performing an act. She is hiding her true intelligence. The only question is how much she is hiding.

[Elegast]

  But I thought that the second of the young vamps was killed.  Meaning Maeve would have had to have been the one sobbing.

No. She was sobbing during attack n°2. SHe was killed during n°3.

[Ms Duck]

me confused. Maeve is a sidhe, a master of glamour. she could have looked like a potted plant if she had wanted too; furthermore, why kill the witness? please explain.

[Griffyn612]

me confused. Maeve is a sidhe, a master of glamour. she could have looked like a potted plant if she had wanted too; furthermore, why kill the witness? please explain.

I'm not truly supporting this theory yet, but if I was, here's how I would propose it.

Maeve did it

When Harry first arrives at SplatterCon!!!, he sees two young sexy women dressed up as vamps.  His first reaction was to blast them.  It was initially assumed to be an overreaction, but may have been his subconscious picking up on Maeve's spell. 
 - One of the two vamps is Maeve in a glamour. 
 - The other is a mortal who doesn't even know who Maeve is, or that she's even there.
 - Even Molly watches them and frowns; Harry assumes because of his reaction.

Two Minus One
After seeing the two together at the beginning, Harry never sees the two together again. 
 - One is in the hallway during the Reaper attack, running in fear.
 - One is killed during the Alien attack.
Where was the other one in each instance?  If they were friends, they wouldn't abandon each other twice.
 - During the Reaper attack, Maevamp might have been the one still in the room, casting the murk.  When Harry hit the Reaper, she ran out, and then the murk released.
 - During the Alien attack, someone had knocked out the emergency lights.  He didn't spot anyone suspicious, but there were people running around.  And with no murk to keep going, she might have been further away.

The vamp's death wouldn't have to be planned, or eliminating witnesses.  It could just be that she was in the wrong place twice, because Maeve was around her both times the phages came through.

Why?
Why would Maeve do all of this?  I don't know, it still doesn't make enough sense to me to reason out.  We can say Mab told her to do it, but we don't know what Mab's intentions were.

[Elegast]

furthermore, why kill the witness? please explain.

Don't know. Harry said that Glau was killed on purpose, so I assumed she was too:

“Glau was barely conscious when it grabbed him,” I said. “It was probably me or Madrigal who was feeling the most tension, but it took out Glau, specifically.”

“You think someone sent it for Glau?”

“I think it’s a reasonable conclusion.”

Thomas frowned. “Why would anyone do that?”

“To shut him up,” I said. “I think Madrigal was supposed to go down for these attacks, at least in front of the supernatural communities. Maybe Glau was in on it. Maybe Glau arranged for Madrigal to be here.”

“Or maybe the Scarecrow went after Glau because he was wounded and separate from the rest of us. It might have been a coincidence.”

“Possible,” I allowed. “But my gut says it wasn’t. Glau was their cutout man. They killed him to cover their trail.”

And that quote:

The two girls went on by, none the wiser, and even Molly only frowned at them and then back at me for a second, her face tilted into an expression of silent inquiry.

I can see Jim grinning writing it.

Edit: I suppose there was a version of the plan where Madrigal took the fall, and no one would ever blame Mab for it. Madrigal could open a Way in a place of linked fear, connected to a place of fear in the Nevernever. Harry sends back the phages, and unfortunately they go to Artis Tor with Molly. Poor Mab is completely innocent. Harry goes to Artis Tor anyway.

The problem is that Madrigal is such a loser, nobody believes he's the bad guy.

[Elegast]

me confused. Maeve is a sidhe, a master of glamour. she could have looked like a potted plant if she had wanted too;

Yes. But that's quite dangerous:

I lunged for a potted plant near me and threw up.

That may have been Maeve.

[Elegast]

Why?
Why would Maeve do all of this?

Mab send her to make sure her plans would succeed. She put the ward to make sure someone was going to die, so motivating Harry, and make him think that some practitioner was calling the phages. Harry would never assume that a faerie was sending them from the NN, and another putting wards. Too improbable.

We can say Mab told her to do it, but we don't know what Mab's intentions were.

Wrong. We have a WOJ on that:

Ask yourself why Mab had Molly brought in.

She had a plan which included bringing Molly through a redirection spell performed by Harry. That level of details needs to be micromanaged.

[Ms Duck]

Yes. But that's quite dangerous:That may have been Maeve.

No, that plant was poor albert the triffid. he allways gets those roles, poor guy is typecast.

Im just missing a few things:

maeve can look like anyone.

does mab need to be in a building to cast spells? Does maeve?

why not just veil?

[Elegast]

maeve can look like anyone.

Yes. And she chose to look like a vampire. Why? She wanted to have fun at the con. Do you think she was going to spend the whole day as a plant, waiting for Mab to send the phages?

does mab need to be in a building to cast spells? Does maeve?

Mab magic is perfect, it does not leak cold. And Harry said "vaguely familiar". Harry had never seen Mab's magic (mind magic, but later).
Maeve plays by the same rule as a wizard.

why not just veil?

Boring, can be spotted, impossible to interact with Harry if necessary. Plus she likes wearing costumes.

[Mr. Death]

No. Maeve, being a Fae, does not work on the same rules as a wizard. She works on the same rules as other Fae, like Mab.

[astelon]

According to this theory what is up with the Scarcrow phage?  Something unusual (even with magic involved) is going on with it.

One minor point: there was no murk in PG.  The lights were temporarily taken out by some kind of hex that also effected the radios, phones and flashlights.  When Harry faced the murk in Small Favor it effected his own light while the darkness in Proven Guilty didn't.

[Elegast]

According to this theory what is up with the Scarcrow phage?  Something unusual (even with magic involved) is going on with it.

No idea.

One minor point: there was no murk in PG.  The lights were temporarily taken out by some kind of hex that also effected the radios, phones and flashlights.  When Harry faced the murk in Small Favor it effected his own light while the darkness in Proven Guilty didn't.

You're only half-right:

“We’ll try mine,” I said, and got the silver pentacle on its chain from around my neck. A gentle whisper and an effort of will and the amulet began to emit a pure, silver-blue light that reached into the darkness around us, burning it away as swiftly as it pressed in, until we could see for maybe fifteen feet around us. Beyond that was just a murky vagueness— not so much a cloud or a mist as a simple lack of light.

“Fire?” Rawlins blurted, turning back to me. “Is this smoke?”

This was more than an hex. There was a darkness spell. I suspect it was the same spell in PG and SmF, but in one case cast by Meave and the other by Mab. Maeve's one was weak and leaked cold, Mab's could be pierced only by the power of Amoracchius.

[Mr. Ghostbuster]

I'm not convinced. To be honest, I think your grasping at straws.

[Agravaine]

I'm convinced.