First, if you're making the argument that a Denarian can have a catch that is satisfied only by the Swords, and nothing else, and it's valid, in your eyes, that the Catch is worth enough to give a +1 rebate on the logic that the Swords are bound to come up because of the vagaries of the story, then why, using that same logic, did you not make it worth a +2 or even a +3? If it's going to be as common a menace to the Denarian as you're implying, using that logic means that it should be worth that much. If it isn't, then why?
Because even though it is going to show up, there are still only ever going to be 3 around. If something is very rare, but once it is obtained, you can arm every member of your group with it, it is going to be more effective. And even if a Knight shows up, once he is down, nobody else can rightfully pick up that sword.
Technically that would serve as a catch, but since it can't be exploited it should be a +0 catch, not plus one because there's a record of it... Not unless the PCs can somehow duplicate the effects of Lady Q's tears and there are clues of how they can do it. Knowing that "unique, one of a kind" thing can hurt a monster is useless if that thing is so rare that the catch can't be exploited.
Clever players might find their way around that.
For example: How about the tree outside the house, that her late husband planted, and she would stand there and weep besides that tree, and the essence of those tears are now in that tree, and for example a spear made from a branch, or a resin-"tear" from that tree is enough to satisfy the catch.
And once this catch is figured out, the Players might be able to provide a sample to each player, and suddenly the impossible catch is gone.
I guess what I am trying to say is this: is there a Knight of the Cross among your players, or are you going to provide a Knight, when your Players face the Denariens? In that case, the catch has a high probability to be met and therefore should at least refund a little bit. Are you planing on not having the catch met at all: Then there is nothing the character can gain out of his catch, it just isn't going to come up, so if he is immun to magic, he is immun to magic, no way around that. Or you want to do a whole mini-campaign out of finding a match for the catch, in which case you might even go around and change the catch midsession from +0 at first (as in: there is effectively no catch) to +1 (there is one (or a few) single ancient artefact, that can hurt this guy, but it has to be exactly that) to +2 (there is something that can hurt this guy, and we have to discover the secret first and get the rare ingredient first, but then we can create weapons to fight him).
The Fae catch for another example is fairly well known and is very easy to come by, so it is very likely that if you put up a fae as an enemy, the players are going to load up on some cold iron before a big fight, so the catch is going to be met almost always, which explains its pretty high refund value. If on the other hand, you are playing a campaign of changelings deep inside faery, and leaving faery is not an option, for this campaign, the catch is pretty much nonexistant and therefore worth nothing.
