1 - The DO doesn't trust anyone; Moridin just gets the most of that small trust. His Chosen are afraid to channel anywhere the DO's cave, and the DO is super-powerful. So any effort to repeat the sealing wouldn't be allowed.
2 - I don't know that the DO can see weaves; so many arguments for or against... Still, I'd say it's impossible for the DO to show Moridin the necessary weaves in the same way that Asmo taught Rand. Shaidar Haran may only be a TP user. Also, SH seems to be more for observation and commands than actual action...
3 - LTT went to SG with 113 AS. Moridin hasn't had access to such a pool of male channelers until recently. But The Dragon's original idea was to use 7 females and 6 males. The dueling secret plans among the AS - the male and female strategies - conflicted, and LTT's ideological opponent got all the strongest women to boycott the Sealing. That alone required him to make a plan B.
Morry still may not have access to a pool of channelers as skilled or strong as the blokes that LTT had access to. Morry probably doesn't have the time to train them to do it, either; he still has to prepare for TG and such. [When the battle erupts, I can't imagine Morry's main move is waiting for Rand to break the remaining Seals - Morry is the true opposite to Rand, as avatars (sorry) for the DO and Creator, so he'll want to be in the fight.] Also, even the DO know LTT's weaves for the 6 men, 7 women plan...
Turning to Saidar, and why he hasn't tried this already... I dunno, honestly. I'd say that they would probably need different weaves to do it - and no one would know those. Lanfear might've been able to figure it out. Still, looking back at the DO's lack of trust, he wouldn't want them to try...
It is a valid question, but it seems unlikely given all the points above.