The Knights of Ozem did not take any actions aimed at causing Altarrin to defect. Nor, to our knowledge, did Aroden. The ring is a Scholar's Ring, a standard magic item which permits reading in unfamiliar languages and which does not cast any mind-affecting spells, nor does it dispel or suppress compulsions. Any magic expert from Golarion can verify that for you, though I'm aware that you'll need unrestricted access to Golarion to meaningfully check this claim.
The Knights have not conducted operations in the Empire outside Oris. Our current understanding is that my headband, which vastly enhances the reasoning abilities and persuasive abilities of the wearer, caused Altarrin to notice that the Empire is caught in a trap of the gods' making, and to decide to Gate to us. He has included his own letter with more detail of his account. We understand that from your perspective this claim is implausible. We would, under the circumstances, be willing to go to considerable lengths to demonstrate its truth, but we possess limited avenues to do that. We've sent delegates, empowered priests of Aroden who I have briefed on the situation and who I expect you to interrogate and quite plausibly to kill (though of course I would be grateful for their safe return, and can arrange to collect them if you decide to leave them alive). This is costly to the Knights and to Aroden, but it is important to me to keep the door open for further talks, even at a steep price.
If nothing else, I hope to maintain lines of communication adequate such that, were your Empire under sudden assault from Tar Baphon, you would regard yourselves as having the option of asking us to come and stop him. I am not the most dangerous thing in my world, and I have no desire to see the Empire destroyed; that is sufficient that we might under some circumstances find ourselves allied.
The Knights ourselves have, at this point, very restricted ability to operate on Velgarth, but in the course of recovering from Iomedae's assassination we contacted other parties which can more easily transit between worlds, and which may conduct operations inside the Empire, in conjunction with Altarrin or independently. The most important such is Alfirin, a powerful mage of our world, whose capabilities in broad strokes are known to the delegates. (The Church of Abadar, god of trade and commerce, has also negotiated for access to Velgarth, but they are unlikely to operate in the Empire or anywhere they are unwelcome, and unlikely to operate in Velgarth at all in the next few weeks.)
From your perspective, this should be treated as notice of the end of the Knights' commitment to not operate in the Empire outside of Oris. We in fact don't intend to, but you don't to our knowledge have the resources to tell Golarion factions apart, and I anticipate that there will be operations inside the Empire using Golarion magic in the near future, and from this point forwards that assurance will probably make your expectations less accurate rather than moreso, which is very much contrary to my intent when I made that assurance.
I imagine my word is, at this point, worth very little to you. The delegates might help with that, or they might not.
But for whatever it's worth, I regret very much that negotiations were, from your perspective, betrayed. I prize the lives of my soldiers very dearly, and those chosen by Aroden are very expensive for Him to replace. I have nonetheless risked sending them to you because I think I have a duty to try much much harder, given what happened, to be credible to you as someone it is possible to negotiate with. I will make other costly expenditures towards that end, if I or you identify some that might be genuinely helpful.
I swear to you that I did not, and would not have, take any actions which from my best guess of your perspective would have constituted a betrayal of the implicit or explicit terms of our negotiations, that I did not authorize anyone to do so, that I sent no deliberate misrepresentations of our intent or our aims, and that Aroden would renounce me, were I to have done what it must seem to you that I have done here.
You can leave future communications directed to me in a scryable location with one of my magic items present for targeting.