I find that confirmation inconclusive, because he explicitly said Moiraine could only remove a few seconds with balefire and just as explicitly said several Forsaken (but not Be'lal) cannot be resurrected. He left open a very large door he could easily have closed, though it's entirely possible that's because no one ever asked him about Be'lal.

The biggest reason to send Taim rather than Be'lal (and I still kinda like the "Be'laim Theory" because saying Taim is not Demandred doesn't mean he can't be anyone else) would be the risk of LTT spotting him by his behavior. Be'lals statements in the Stone indicate he and LTT knew each other more than slightly even before the War of Power (which, as has been noted in earlier threads, might explain why a "Be'laim" would feign clumsy disinterest in swords). The need for the masquerade is the only reason for Aginor to accept a position of subservience to Taim--yet there's reason why he would do so even among fellow Darkfriends if Taim were not one of the Forsaken. It's entirely possible that he was ordered by Moridin to do so, or that Taim has been "promoted" to nouveau Forsaken, but those aren't the only options. As to why both would be sent to the BT, it makes sense to have a Plan B among a group consisting of every male channeler Rand can find, just as it makes sense to have more than one Forsaken to watch each other as well as Rand. Moghedien wasn't the only one minding the store in Ebou Dar, and Be'lal wasn't the only one on the scene in Tear; in both cases circumstances drew a crowd as surely as the DOs orders, but that should apply to the BT as well.

If the DO didn't have plans for tools of as much potency as Graendal (and Moggy, and Lanfear) I doubt he'd keep them around at all; charity isn't exactly his hallmark. Yes, Be'lal has very little characterization; he was present for such a brief time we didn't have the chance to learn much about him and, unlike Aginor and Balthamel we haven't (knowingly) seen him since. That seems a little too convenient; despite his early appearance we don't know more about Be'lal than we do Demandred, making him a very odd duck indeed among Forsaken. It would be rather abrupt to suddenly pull Be'lal back out in the last book as someone we haven't seen or heard from since TDR, but if he does return I expect it to be as someone we recognize. That doesn't have to be Taim (though it would explain a lot, except for the fact that Bashere IDed Taim and we don't know of any disguise weaves that can withstand touch) but finding out that someone we've been watching for half the series is really Be'lal in disguise could prove a very satisfying surprise in a final volume where most fans seem to think no surprises are possible.