Many of these whistle-blowers are older experienced engineers who will be biased towards a higher death rate.
This, plus being highly involved in any court case is extremely stressful, which can take a toll on your mental and physical health.
Which is why I'm still kinda leaning towards an actual suicide with the first case. Being stressed, tired, having your life dictated around court schedules while you sleep in hotel rooms....... I could see that wearing someone down after a while.
I just don't think it makes real sense for a company to hire an actual hitman to operate in the US. Corporate murders happen, but usually overseas, and usually not when they've already testified.
Not saying it isn't a possibility, I just think it'd be cheaper to pay the guy off and have him sign an NDA.