Why would the Finn only take a little part of Lanfear's power, especially if they also killed her then.

Besides, as BS said, "Cyndane is not under any shenanigans. What you see is what you get." This makes pretty clear that the Eelfinn didn't change her "conduit" with some ter'angreal, because that is basically an explanation like the TP shield-theory and should be reversible (though I anyway don't see why the OP strength would be a bit less, if you could contract this conduit somehow; if anything it would maybe just take a bit longer then to grasp all the possible OP).

All in all, it seems most unlikely that the Finn are responsible for Cyndane's somewhat reduced OP strength. It makes more sense that it is some effect of the transmigration (as said, likely the incapable body), or maybe the Healing of Stilling/Burning out (e.g. with the TP).