There is some unnerving implications about the fake Lord Arundel (who's heavily implied to be a disguised Thales) being considered in-universe as another instigator of the Insurrection of the Seven, despite the fact the first act the real Arundel did during said event was fleeing along with his niece to the Kingdom for the sake of her safety. The way both Hubert and the Church's books talk about Arundel and Duke Aegir's alliance, 5 years after the Insurrection had taken place, make clear that, to give the impression Arundel had always been on board with the plan, they had to have teamed-up nearly instantly once the former returned to the Empire with Edelgard, which suggests both had been likely in contact for some time already. What makes it even worse, is that in Lysithea's A Support with Byleth, she mentions that 3 years prior to the Insurrection, she saw Agarthans working along with Imperial staff for their crest experiments in Ordelia territory in plain sight, and given her words in her paralogue that those who fled from Hrym (which was under the Duke's control back then) to Ordelia were forced to return in the end, ultimately implies and corroborates that, not only the Duke had been in contact with the Agarthans for quite some time already, but that he is more or less single handedly responsible for allowing them to gain so much foothold in the Empire to begin with (and by proxy, eventually sending Fódlan into ruin). It makes me imagine ferdinands reaction to that
I am absolutely certain that Duke Aegir was dealing with the Slithers for a long time. I don't think it's at all an accident that he was given control of Hrym, which borders with Ordelia, right before the experiments on Lysithea and her siblings started. It was pretty well happening right under his nose.
And it makes sense that the Slithers were involved in the Insurrection, right? Even if some or even all of the nobles behind the Insurrection thought that they were acting justifiably in response to the emperor's overreach of power, the fact that it directly and immediately lead to experiments on the emperor's kids sounds to me like that was sort of payment to the Slithers in exchange for their support.
I think Ferdinand would be pretty horrified, tbh! He knows his father has done bad things, but I don't think he's remotely aware of just how far he'd gone.