I think the risk is acceptable, because Demandred would probably never learn about it, before it is too late (the order to kill Rand is added by "that it would be better they died than let themselves be discovered again. By anyone, even the M'Hael, as if he did not know of Taim's order" ). Even if Demandred learned about it, he still had no proof that it was Aran'gar.

Demandred called the DF Asha'man and Osan'gar, who attacked Rand in Cairhien, "blind fools" and "idiots". That doesn't really indicate that he sent them to attack Rand in Cairhien, but that Demandred learned about what they did and considers their attempt idiotic. And the command to kill Rand after in Far Madding from "Demandred" came after this failed attempt.