Should Be'lal in Taim's body posing as Sammael sent an Shadowspawn army against Rand, if he can attack Rand with his cronies?

It makes more sense to me that Graendal posed as Sammael (who she knows is dead) and sent the army to possibly weaken Rand & Co. before he might go to Arad Doman (the logical next step, which might also have been reported to Graendal via her spy network). Plus, it's likely that Graendal would play such mindgames with the other Forsaken, who might become uncertain if Sammael is actually dead. Furthermore this move was certainly also done to make Demandred the obvious culprit in Moridin's eyes (this becomes especially clear if one notices Graendal's and Demandred's reactions to "Sammael's" appearance).

In any case, I think one can't seriously dispute that Be'lal being Taim would just work, if all other Forsaken (including the Nae'blis Moridin) don't know about it. And that simply doesn't make much sense, IMO.

I can definitely see Ishy train Taim and therefore Taim adopting symbols or phrases of the Shadow . Ishy also offered that to Rand. Plus, Ishy probably was also involved in converting Isam to the Shadow, after he reached a certain age (IMO, the infant isam was raised by Farstrider & his woman before).