It depends on the varience of strength amongst the Aes Sedai. And Suine said half the women in salidar outranked her now, implying that half didn't. She may not be as weak as you assume.

 

The simply fact is that Lanfear and Moiraine were both channeling extreme amounts of the One Power as they passed through a ter'angreal that subsequently exploded. At that exact moment Moiraines bond with Lan was severed, which we have only seen caused by death or stilling (and not by the intentional release of the bond--that doesn't cause the death feelings). At the same time the next time we see Lanfear she is weaker then she was, again something we've only seen happen when a women was stilled. Finally the fact that it would be extremely hard to keep a channeler prisoner--especially one with a knoweldge of the Finns, and the gateways back into our world from theres.

You know ... the possibility of Moiraine and Lanfear both being stilled seems very likely ... and it would explain a great deal about why Lanfear died and Moiraine is still captive. And it is a much more plausible explanation of why they simply didn't kick the Eelfinn's trash, in light of your very valid points about the foxhead ter'angreal.