Even if true, the forsaken where there when LTT made the seals. Their presence was why they were trapped in the bore in the first place. That means that Moridin must know or atleast have a pretty good idea how to replicate what LTT did. I figure he likely saw the weaves since LTT would not known about the taint and would have had no reason to waste time reversing(or was it inverting?) his channeling.

And that is assuming that spies and traitors did not inform him and the shadow of what LTT was planning. I mean we know LTT suggested his plan repeteadly to the other AS. Presumably he must have described it in some detail before they voted on it or whatever. Given how publicly that must have happened it seems likely that the shadow knows what/how LTT was planning on doing the sealing.

So basically, Moridin/DO should pretty much know already how to do a close enough job to allow the DO to taint saidin again, or atleast they would be able to figure it out with a bit of research and study. So once the 3 final seals come down, DO/Moridin should logically be able to taint the OP as a whole.

Now I suppose you could argue that there was something special that happened during the original sealing that cannot be replicated. Like for instance maybe LTT as the "creator's champion" has some special status or ability that allowed the DO to touch saidin through LTT in a way that is not possible with other channelers like Moridin. But thats pure speculation on my part.

If we don't assume anything like that(that LTT/sealing/taint was utterly unique), then Moridin/DO should be able to taint saidin again. Maybe not on the first try, but since Moridin can hang around at SG and channel freely, he can just try and try again until he replicates the correct conditions. Sooner or later, he would get it right and BAM, tainted saidin. And ofcourse once he had managed that, it should be relatively simple to repeat the process for saidar.