Maize Also and I loathe to say this with every fiber of my being The mother is a potential candidate for the second gathering attack and I say this because she hates Incubus kings and That she has a divine shard.
Ugh. Hadn't thought of Mother. She has the power, but .. I doubt she has the intelligence, subtlety or the interest in Ivanan matters. She and her Guard generally seem pretty hapless, unable to even clean up a seed of corruption without outside help.
I still just don't see Fheliel being a real suspect, sorry. Not only is there no indication she really has the personal power to pull it off, but neither is it established that she'd have a real motive. Nothing shown to us so far gives us any hint that Fheliel has divine powers. Honestly, the Anak seems more likely in both dimensions.. he clearly -could-, and even has some reason to - he wants to keep IKs off-footed so that none of them disturb the balance, for example. Not that I think it is likely to be the Anak, but he's at least in the running, and I don't think Fheliel is. Fheliel is busy running Ghenalon, I doubt she has time (or power, or inclination) to try to be a fake Ivala.
Honestly, more likely than any of these, I think, is a character we've seen but barely know. Someone who has been mostly in the background. Perhaps someone in Alanon's council, disgusted at what they perceive to be his failure to act. The Regent perhaps? And perhaps this person has been .. influenced by Lillith or some other power (Tarunak is in the running for this role), who gives them power but at the price of increasingly chaotic decision making. Then the appearance of the Crystal can be seen as an (over)reaction to the challenge the Fucklord posed when he did his Arclent-wide projection. The Chosen always act against what such a person may perceive to be threats to Arden authority. This would actually fit.
Then the real culprit would be whoever is driving/maddening/enabling/manipulating this person to act. The neat part is, just as in Tarunak's pitch to Trin, he could empower this person to act and -not- direct his or her actions, if he thinks their actions will be entertaining. If so, then the Chosen attack still could have been a surprise to Tarunak.
This may be a longshot (someone like the Regent, but Tarunak power-charged), but it actually fits what we've seen. Chosen becoming both more powerful and erratic, the appearance of the Crystal, etc. Tarunak would not bother to try to control or predict this person, just amplify both their power and their insanity and let them run amok, just to see what they do.