I don't think we can safely say that he can't resist her so much as he doesn't even try. It could be he feels an obligation to do it. Remember that when we first see them, Flemeth is kneeling before him. It could just be she is merely trying to be at eye level or she could be a sign of deference.
Also, Morrigan could hardly have predicted Flemeth would find them after a decade.
Sure, but it's the same difference though, within the context of this conversation, Flemeth's motivations, good or bad, don't actually matter - to Morrigan. Whether he can't resist (which he does somewhat imply through dialogue as I recall), doesn't try to or is simply being tricked/manipulated by Flemeth (a master of the craft) or if it's something else entirely the result is the same. As the scene is framed, he's at Flemeth's side, even after embracing his mother, it's to Flemeth's side he returns, so at least as far as Morrigan is concerned whatever the circumstances are, his current situation is dire/undesirable in her eyes, hence why she has her almost in tears, falling to her knees moment. Though you can certainly make an argument that Kieran was not actually in any real danger as it was just a lure for Morrigan so Flemeth could test her while also grabbing the OGB soul.
I agree that she couldn't have known that Flemeth would find her- to an extent, which is why I've brought up the question as to whether she's grown complacent simply from having a happier/easier life over several years and is overreaching through hubris in that moment. However, circling back to an earlier point, Morrigan is charged with looking after Kieran, thus any actions that she undertakes which may adversely affect her ability to do that should give her pause.
Don't misunderstand, I think it was an irresponsible action on her part. Still, it's interesting to ponder whether she truly believe he would be better without her and thus it was a selfless action or if she was merely trying to justify her willingness to abandon her child making the choice selfish.
No, upon reflection and as outlined in my recent post on the self-doubt issue, I don't believe she really meant those words, but it's not really the words (regarding Kieran) I have a problem with so much as her actions in the deliberation section itself, to me it feels that it was someone's goal to remove any and all obstacles from Morrigan putting herself forward as a drinking candidate. To do that, they had to come up with a reason why she would potentially put her son (and Urthemial's soul, another unique prize to preserve) in danger. Secondly, you can literally tell her that she will be under a geas (which in essence wouldn't be far removed from possession, which Morri wasn't too keen on), this combined with all the other clues in the Temple of Mythal and in conjunction with the fact that Morrigan's purported expertise in elven lore is at least on par with an elven inquisitor (whom, like Andres Hendrix said on the previous page, could make a reasonable guess as to Flemeth being Mythal), the potential risks should have been more apparent to her and given her pause. Even more so if Abelas leaves unharmed, bearing in mind his parting words. That's my real issue with the scene, it's a kind of "but thou must" situation for her and if I'd seen her have a sliding scale of willingness to take that risk based on prior choices, I'd have had a lot more respect for the scene.
It's not a major deal breaker, but since she was handled near-flawlessly everywhere else, it sticks out to me. Since one can make a strong case for childless-Morrigan wanting the Well, it appears to be an instance where "generic" Morrigan wins out over the more nuanced variants. I can buy Morrigan wanting to partake in the Well when the whole venture starts off, to have something within her reach that can answer so many of her questions while feeling that she is doing some good in the world, I understand the temptation but as more and more questions are raised and you get to the final culmination, after evaluating the risks, I'd say her choice should have been different (and it should *never* fall to a secondary PC with no connection to her to talk her down in any case - that is meta-gaming).
But, Morrigan drinking does get her the Dragon powers (which I love so very much) and the dev notes do imply that the geas effect would be cancelled out... Like I said, it just sticks out, especially with what happens and what she does/says immediately after it, but that may indeed have been the intent, so as to better underline the fact that Kieran is important to her in the next scene and punctuate that growth with her refusal to agree to her mother's demands.