We know that the Shadow searched for male channelers for many years before the series, and it makes sense that Ishy would eventually find a willing pupil, whom he could teach, like he also offered to Rand (it's also a bit like Isam's raising). This would explain Taim's knowledge, etc.

Especially that Taim had the Seal strongly indicates to me that he already followed the Shadow, when he joined Rand. Probably he got the Seal from SH, who also ordered him to join Rand.

I agree however that Demandred clearly wasn't involved with Taim or the BT (except for having a few spies there apparently). He has other power bases, especially Murandy now.