It is a valid question actually from the OP.

The DO was there when LTT sealed him. He must know what happened just as well as LTT does, and quite possibly better.

Why cannot he simply teach Moridin the necessary weaves and so on to basically replicate what LTT did and touch the DO with saidin the exact same way. Presumably that would allow the DO to once again taint saidin.

I mean even if he did not know about the possibility of the taint and how to do it back in the AoL, he must certainly know it now that he has already done it once. And ofcourse presumably tainting saidar would work pretty much the same, except you would need a female channeler. And while some of the females might hesitate, Moridin can just order one of his mindtrapped slaves (Cyndane&Moghedien) to do it and they would have little choice but to obey.

I suppose it is possible that the current seals prevent this from happening right now, but if so it only means that Moridin would be able to replicate what LTT did and retaint saidin the moment those final 3 seals break.

And ofcourse the new seals (if they are even necessary for the taint, Moridin could just abort right before the sealing was finished or alter the procedure so that no sealing happens at all) could be made of regular steel or something which could easily be broken afterwards to re-release the DO.