We know Moiraine's bond to Lan snapped, which we have only seen be caused by death and severing, and we know Moiraine isn't dead. We know ter'angreal are dangerous things that can kill or burn out the unwary, and this one was malfunctioning violently. We know Lanfear was held by the Finns for a time, and that she came back in a new body, weaker. We know the Finns can't give a new body. The only thing we know that can cause a drop in strength like that is being Healed by a woman. So they were burnt out by the ter'angreal, held by the Finns, Lanfear died and came back as Cyndane, and was weaker when Healed. Evidence. Like I said.
The first half of that has nothing to do with Lanfear at all. The rest still fits the alternative theory, so I don't see as how that disproves it.
You seem ready to call out the 21 level list as just a theory with substantiating evidence, but when that "evidence" fits your own theory (or one you support), you think it is "proven" when it isn't. It's just a theory that fist the facts, but has a few holes (strength issues).
I'll run it through for you. Mierin goes to Finnland in the AOL (by the TOG, Portal Stone, or even via T'A'R in the flesh). She wishes for max power/max beauty or both. She re-enters for the second time when Moiraine knocks her through. The Finns are pissed. They take back the gifts they give, her strength returns to Mierin level (which is still strong). She either dies later at the hands of the Finns, or the DO assassinates her (via T'A'R, Slayer, whatever) in order to free her and reincarnate the body.
Now, if there is knowledge out there that makes the theory impossible, please share it. Forgive me, I don't have 3,000 posts. And I'm open to multiple ideas for how something could happen (unlike others on here, apparently).
Both "theories" have their issues. But either could be true.
Healed by a Woman Issues:
The healed by a woman theory has an issue with the strength drop. The absolute strength drop is the best explanation, but is has problems. Namely, what if a weak channeler (Morgase) or the weak sister from Cadsuane's group got stilled and then healed by a woman? Would they be unable to channel? As the "absolute" strength drop has now taken them below zero? But I think it IS possible. And I'll reiterate, doesn't perfect female form and maximum strength in the power seem a little fishy?
Some evidence to support the fishiness:
When Cyndane feels Alivia and figures out she's stronger in ACOS at the cleansing, she thinks (something to the effect of) "she was stronger than Cyndane before the she was held, but no one could be stronger."
Point being, how does she know "no one could be stronger" absent some wish to be as strong as one could be? How could anyone know no one could be stronger? If the DO could do it, he'd do it for all his channelers, wouldn't he.
Again, if this theory has been disproven, please share with me the evidence that does so, so I (and the thread) can move on, instead of spouting vitriolic statements that it has already been disproven. Without the actual evidence disproving the theory, to me it just looks like circular reasoning.
Otherwise, I think you have two theories that still fit the facts.
The only problem I have with this theory is that Matt tried several times to go through the door again and it did not function for him, I do not have an exact reference from the book but I could swear I remember him trying to go through again.