If we make one major assumption without any evidence to back it up, I believe Ialai moves from "Plausible" to "Highly likely."
The assumption I would make is that Ialai knows Adolin killed her husband but can't (or won't) prove it (yet). As I mentioned, I have no evidence to support this assumption, but it seems like something possible for her character. She has a network of spies. She probably knows (or can guess) at the enmity between Adolin and Sadeas. I believe she can probably put it together, but there's no proof that she has.
If Ialai knows who murdered Sadeas, it makes sense to me that she's behind the copycat murder. She would probably assume (correctly) that Dalinar's response to the 2nd murder would be to appoint someone to investigate it, and she'd also probably assume (again, correctly) that person would be Adolin. She then has a couple more copycat murders performed (among Dalinar's allies) before providing proof that Adolin killed Sadeas. Everyone then assumes that Adolin is behind the other murders; Dalinar must distance himself from his own son; Dalinar's allies distance themselves from him. All of these outcomes line up with what seem like reasonable goals for Ialai (e.g. see her husband's murderer shamed and brought to justice and see her husband's enemy cast down).
Again, there's no proof Ialai knows Adolin killed Sadeas. Maybe she doesn't actually know Adolin did it and just believes she can pin it on him. Either way, I believe she stands to profit the most from Adolin's appointment as investigator for his own murder, which leads me to believe she's behind the 2nd murder.
Now to move from "only slightly crazy" to "absolutely insane." Ialai probably believes Adolin is an easy target because she's smarter than him (not that Adolin is an idiot, but there have been a couple mentions of him not keeping pace mentally with other characters). With Renarin's future sight and Shallan's lightweaving, Adolin does have two assets that can help him out. His best outcome would likely be admitting to murdering Sadeas (and accepting punishment for it) while Ialai is proven as the mastermind behind the others. Does anybody actually see Adolin getting out of this without accepting punishment for murdering Sadeas? Punishment could easily be a loss of rank, loss of blade and/or plate, jail time, or a combination thereof. Now, I've been a proponent of Adolin not having a future Nahel bond because I didn't believe he was broken enough for one; however, if the plot plays out like this, I can see the stress of investigating his own murder and the shame of his punishment leading him to be broken enough for a Nahel bond.