"Your last letter was fascinating! You have proven me wrong, once again, by doing the impossible. I shouldn't have doubted your resolve, and I hope you will keep me apprised of further progress."
I don't think so.
Right. Which is why the Seekers should have made an effort to find out. That's the point.
Right, and that letter specifically confirmed that what Orsino is praising was the Harvester where? Or that Orsino got explicit knowledge of Quentin's atrocities where?
People look at that letter and apply the Harvester to it, but that's speaking more from what people think then objective facts.
Here's what we know. Quentin was obsessed with bringing his wife back from the dead, so much so that he left the Circle of Starkhaven (as it's implied) and began working on it. He contacted Orsino and requested tomes to help with his research into bringing the dead back to life. Whatever those letters Quentin sent entailed is a mystery, but Quentin -- crazy as he is -- wouldn't have just written "So I pulled a knife on some broad, killed her, hacked her apart, bound a demon to a trinket of hers, and now have gotten some twitching movements in her limbs. PROGRESS!!!".
If anything the man would've kept his research reports detailed to a minimum and not talk of the more unsavory aspects so that he wouldn't alienate the man who described blood magic as "dark arts" (not just to us, but to Quentin himself) from his side.
Literally all we have from that letter is that the two were in correspondence together. Anyone who says Orsino knew for a fact what Quentin was doing is injecting their own bias into the equation.
Frankly I'll believe the man who, when push came to shove, no matter what says the same thing: that he didn't know what Quentin was doing until it was too late, by which time he was between a rock and a hard place. The man's about to off himself in the end. Keeping the truth hidden is at this point not something he cares about doing anymore.