How come there are conflicting reports on what they look like? I'm no expert on lore, but at one point, an Akaviri army tried to invade Tamriel only to get defeated by Mr. Reman. Now, with Reman pardoning many of the Tsaeci invaders and incorporating them into the Empire as loyal citizens, plus the fact that most of Tamriel were ruled by a Tsaeci at one point, there would be a clear and universal agreement on what they look like. So what happened?

Maybe all the reports are true and they are just a race with different forms like the Khajiit and the Argonians.

I'm fairly certain they are snake-men in line with the official description and the description in 2920. The new book from ESO, http://www.gamesas.com/topic/1455331-the-worthy-ar-azal-his-deeds/, also mentions a "Snakehead Potentate." So I'm guessing they are officially going with that description.

What sources say they are humanoid again? Just Alduin's Wall? (probably not actually Tsaesci).

I think people are also thinking of the whole pale pass quest in Oblivion which was due to the engine not being able to make a npc that looks like a true Tsaesci and it also supports a theory that "eating" the men of Akavir could mean they assimilated the humans into Tsaesci culture.

I believe Disaster at Lonith did mentioned that the Tsaeci army appeared to be "mounted men.". If I were to guess what that means, Emperor Uriel V's army was fighting a humanoid army that was primarily using some form of calvary. And since it was written by a government official, it could be a more trust-worthy source than most others. Other than that, I don't know of any others describing them as humans.