I don't think that we can look at Lan as being typical of what would happen to a warder if his Aes Sedai died. He was at war with the blight before he met her, and would go back if she died. Somewhere it is said that if an Aes Sedai is killed, the only way to save the warder is to put him into the bed of another woman. (ok, I'm going to be really vague on references, because I am reading the series through for the third time and it's all a bit jumbled right now) So, Tam _could_ have been a warder that survived the death of his Aes Sedai. Now, he could have become a blademaster withought being warder trained. Two examples would be the seanchan that Rand fights and the Carheinin lord that he fights in CoS. As matter of fact, I thought that somewhere in the first three books Moraine said that Tam became a blademaster while serving in the Illian army.