Even if they're "immune" to OP weaves, Moridin channels the True Power, and RJ gave us all the hints the DO is actually more dangerous under Sindhol's radically different natural laws than in the human world. The way Moiraine speaks of the events (with Moridin being paraded around to Moiraine's location etc. and the fact we see that the Doorway in Tear has now been destroyed, it seems obvious that Moridin went in there protected by the treaties.

I don't understand your objection about how Moiraine and Lanfear went through the Doorway. It's totally irrelevant. We know the OP can be drawn from there, from Rand (who never tried to touch the Finns with weaves, however). Moiraine when she got there first was led to the Chamber of Bonds and had her Bargain, she then got screwed because it "wasn't the Eelfinn's problem" if the doorway by which the treaty forced them let her out was destroyed, and she got captured, put to sleep and they woke her up periodically to be drained, frightening her with lies they had killed Lanfear by draining her too fast (we now know from Moghedien it's a lie and it's Moridin who killed her or demanded the Eelfinn killed her, it means the Elfinn have a very mean streak). She wasn't all the time inconscious and obviously she didn't manage to get out with the OP. It's even more obvious with Lanfear, as Moraine could have decided not to fight them... but Lanfear even if she was inconscious some times, they still managed to hold her. Unless you believe Lanfear didn't try to fight being held or drained whenever she was conscious if she could.... They have the means to make ter'angreal stopping direct weaves, and to make ter'angreal portals (rather obviously channelers can't make gateways from there work, otherwise it's the first thing Lanfear would have done).

It would make sense Aginor would try to use genes of Sindhol creatures if he managed to capture some. He was aiming to produce the ultimate predator, and the Finns are obviously extremely dangerous creatures, with interesting powers. The pinnacle of his experiments with Sindhol genes may have been the gholam: immune to the OP, able to use portals/gateways.

Oh well, this is a question Brandon is more or less likely to agree to answer if it's put to him (unless it's the kind of information from RJ's notes about Shadowspawn Harriet wishes to keep for the Encyclopedia).