All the following is total speculation on my part - please feel free to poke holes in my theory:
Some of the intial reports said that it was a friend/coworker of Janet's who first alerted police that something was wrong because Janet hadn't responded to an (e-mail? instant?) message online. The only plausible way that makes sense to me is if Janet had told the co-worker, THAT day at work, that she had found something out (total guessing here, but something along the lines of he promised to be faithful when we reconciled and now he's carrying on with someone in the office?) and that she intended to tell him something that she knew would anger him (that she was leaving and taking the baby, perhaps?) that night. That she was concerned for her own safety, prompting the co-worker to be concerned as well.
Janet worked in an office, so I assume she arrived home in the evenings around 6:00 or so? I doubt something of the magnitude of "I'm leaving you" could wait very long after she arrived home. She tells him the jig is up, he tells her she's crazy, and she goes into the office - where the laptop is on the desk - to show him her proof. She's sitting in front the computer when he stabs her in the shoulder. She gets up and fends off another blow with her hand before he stabs her in the chest, fatally.
Her husband supposedly left the house at 7:30 to go play soccer (as someone on here has suggested)? That time would fit - off he goes to play soccer as if nothing at all is wrong, establishing his alibi. Her co-worker sees that she's online, tries instant messaging her to see how it went and gets VERY concerned when Janet won't answer.
If Kaiden was asleep when all this happened, I don't think it totally implausible that the husband would leave him alone.
If Janet was using a laptop that night, and the laptop was in the bedroom/office where her body was found, and that laptop is now missing (either hidden away somewhere by her husband or taken in an actual burglary), isn't there some way law enforcement can track whether or not it's being used?
And why hasn't the concerned co-worker come forward (assuming there is one, of course)?