imo, i feel like that is very improbable
your reasoning is also a bit flawed...
As mentioned in the OP, Tri-Tachyon were on their way to deploy planet killer on Chicomoztoc. They were ready to murder hundreds of millions, basically most of the population in Persean sector, just to win a war. They are, by all means, the most likely suspect for destruction of Opis, and the arguments in OP all check out.
that might be a bit of a nitpick, but this specifically happens because "new guard" takes over,
Here's how it's stated on the accurate history page
Spoiler
(https://i.imgur.com/mDFC898.png)
Forlorn Hope: The description conspicuously mentions a phase cruiser carrying a planet-killer device. While it could, in isolation, just serve to characterize Tri-Tachyon as amoral people who are willing to kill hundreds of millions from orbit to win a war, the explicit inclusion of the information that the PK-carrier was a phase ship, combined with the focus on the mysterious and unattributed destruction of a planet later on, feels calculated.
I also love the factual inaccuracy of stating that the Opis occurred after Forlorn Hope and not before
Spoiler
(https://i.imgur.com/qPlUrzx.png)