Oh, neat! I hadn't read this, but my hypo is literally case (3). His answer: "who knows?". Excellent.
In my hypo and his, the origin of the private information I've acquired is still business my employer has conducted, so there's a better-than-baseline probability that the SEC would see this as misappropriation --- even if my employer wasn't directly going to trade on this, or if it was hard to trace any harm to my employer, it's still potentially not OK for me to profit from it.
The thing where it gets murky though is that the theft argument needs someone to be stolen from, or defrauded.
In classical insider trading, the victims are the investors in the M&A target. I'm misappropriating MNPI about the target, so I'm essentially defrauding the target's investors.
In the "shadow" case, I think it's a harder argument to make. If I work for a potential Cisco M&A target that falls through (as per the GPP), I've got, MNPI about my org, and probably MNPI relating to Cisco. Can I defraud Splunk's investors with that?
Or Matt Levine on the same case: https://archive.ph/5Dlmw