Unless there is no OGS, in which case Flemeth is seen doing the exact same thing. But I want to go back to Solas. What indication is there that he absorbed her powers. It looks to me like all the blue energy is leaving Flemeth's body and is going into the eluvian.
On a side note, I think the OGS was intended for Mythal, honestly. Morrigan was also intended for Mythal, probably. Or Flemeth would have used her and carried Mythal with her. And if OGS had been intended for Morrigan, then why didn't Flemeth just wait and have Morrigan perform the extraction? Or why didn't she teach Morrigan how to do the extraction when she taught her the ritual?
Um... no, Mythal has control over Morrigan. I think that was firmly established. Mythal controls Flemeth, and Mythal controls the one who drank.
Uh.... either you're confused or I am, because most of this post is word salad to me.
When you say "the OGS was intended for Mythal", you do realize I was talking about Mythal's OGS... right? That's the one that's hypothetically being passed from Flemeth's body to Morrigan. Kieran's (Ghilan'nain's, most likely) OGS is a separate matter. Since BW is under a bit of pressure not to let the story diverge too significantly, it's likely that OGS is currently being stored away in the fade, since that's where it would have ended up anyway if your world state didn't include the Dark Ritual.
And yes, although it's ambiguous whether the last bits of Flemythal's essence are passing into the eluvian or Solas at the end of the scene, it's not ambiguous at all that he seems infused with a new and similiarly-shaded power when he lifts his head from her drained body.
In summary: Mythal's OGS can control whoever drank from the Well. If Morrigan drank, then when Morrigan's spirit merges with Mythal's (assuming she consents, which seems likely) she'll essentially be holding her own leash, nullifying the effect of any geas.
...but things get quite a bit trickier if the Inquisitor drank instead.