Never considered if Onionhead was related to the UAs, the UA being from the opposite direction to Onionheads original world
The bombing of Panem was conducted by Admiral Vincent, Hudson's ally, as a way of undermining Halsey's Authority and support.
Given the original S&R of Starship One failed, and the independent investigation of the wreckage failed, we may never know the truth.
With the ease at which Halsey could have suffered a further accident when her life pod was discovered, I doubt she has any information that will threaten Hudson's position.
Miraculous high profile survivor with now limited political support and power, seems like a poor choice for an Agent be it Thargoid or otherwise.
This isn't about Thargoids, it is about a shadow faction like the INRA or Dark Wheel showing their influence by eliminating or incapacitating
key figures in the political landscape, while shoving it under the table of other politicians they have allready intimidated.
Its an old trick from the books