And all of them seem to know exactly what a seal looks like. Rand, has had several seals to compare between. One would guess that his high intelligence combined with his experience would qualify him as a expert on the subject.

I can't imagine, due to the fact that Cuendillar is so rare, that there are many counterfeiters of the material roaming about. Fewer who might be willing/able to make a copy of the seal.

The likelihood that this is a fake seal seems slim to me.

I think that the Shadow willingly sacrificed this pawn in order to get Taim in. it was a worthy trade, since the seal seemed to be generally worthless to hold onto for the shadow. After all, they are breaking on their own.

This was of course before Rand had started holding his discussions with Fel and was told that he might need to clear away the remnants of the old seal. This was the point I believe that the Shadow once again became interested in the location of the seals. (But it has been a while since I read that part of the story)