Some thoughts:
1. I think Mira's theory is the most likely. Peabody mind controlled/influenced someone, probably a young someone, to lower the wards, and that's how the Red's were able to get in.
2. Elaine doing it is possible. Here is how I see that one. Elaine and Harry wouldn't just know how to open Simon's wards, but they would know the theories and techniques Simon would use to set up the wards. With this knowledge, they would be able to figure out how to open his wards. It would probably take some time and observation. Isn't there a statement that given enough time, just about any wizard would be able to figure out Harry's wards?
3. Another possibility is that Simon is Cowl. This has a part A and part B.
Part A is that the vamps got in because Simon was in the Circle with them, and they gained entrance that way and betrayed him, they gained knowledge of his wards that way, or they gained an agent who betrayed him that way. The agent could be Elaine.
Part B is that Simon orchestrated the whole thing for "reasons."
4. If Simon is Cowl, it is reasonable to believe that Justin was bad from the get go. Simon knew all about Justin's Elaine and Harry project. After Elaine disappeared, Simon finds Elaine and continues her dark training/subjugation/whatever. He then places her in the Summer Court as an agent.
Some other thoughts:
No, one has to look at the long term goals of whomever was backing Peabody. In Luccio's case the goal was to turn the Council against the Wardens, also to knock out the two most effective Wardens.
Murphy suggests that LaFortier had stumbled onto something that would lead to Peabody finding out, so he acted out of desperation to cover his tracks.
There is no indication that Peabody is merely a slob.
He was in fact fastidious.
After she was shoved into a younger body by Corpsetaker. The SC could only be tweeked not bent due to their age.
This is correct. It wasn't that Peabody was powerful enough to manipulate the powerful Luccio. It's that Luccio's young mind was supple enough to be warped without breaking.
In my opinion Cowl is Cowl, he isn't Simon or Justin or anyone else but himself.
He's definitely hiding his identity, so he is someone. Whether or not he's anyone we would know or care about is up for debate. I think he is either someone we know or someone we will know for Doylist reasons.