There could be another possibility. Someone else stationed at Archangel is Cowl... That they betrayed Simon, and aided the Red Court. When it was said that Simon had defended himself from a Death Curse before, perhaps it was Simon's. Simon targeted multiple targets, Cowl could have been one of them.
I hadn't considered that. Interesting idea, and perfectly plausible, though narratively it'd be kinda awkward to explain, I think. Imagine Harry uncovering Cowl's identity: "I know how Cowl is. His name is...Steve. Steve Crunderman," and then go on to explain his connection to Simon. It's possible, and it'd be fun to see Harry kind of stumble through an explanation like that.
Here's a question: what was there to know about the Reds that the loss of an expert was really that significant? Everyone knows their main weaknesses; faith magic repels them, sunlight unmasks them, decapitation and blood-bag-stabbin' takes them down. It seems like anything he might know that would be relevant is stuff the Fellowship would, too—locations of Red strongholds, common traits and hunting habits, stuff like that.
Not to speak ill of the fictionally dead, but I think Simon was just padding his resume, like I totally didn't do when I worked as a consultant.