Was the black announcement stone not there pretty early? And presumably Taim would be the one that made it (got SG rocks in other words). So would that not confirm that Taim was always a darkfriend and was not recruited or forcibly converted later. This leads me to wonder why the heck he gave Rand one of the seals, if he was already loyal to the DO at the time.

Moridin atleast wants the seals found, and breaking them on his own schedule is a vital part of Rands plan. Rand needed a teacher anyway, so there was actually no reason for Taim to give it to Rand. So why did Taim give Rand the seal, rather than keeping it for himself or giving it to Moridin to gain extra prestige.

Hmm. Unless he was recruited in the small time period between Rand making him a teacher and the construction of the announcement rock thing. I guess immunity from the taint would have been an attractive offer at the time.