Being completely honest, I like both! I've always been someone who is convinced of a fanon idea by how the fans interpret and work with it, anything well written/show can convince me, and so far I've seen good use of both ideas! (People tend to lean more towards the "Tenna didn't know" tho).

If it is for myself and the artworks I do, I would like you guys to think about it as "Tenna did notice the changes and the possibility of the trio not being the real Mike but his spiraling mental state got him in denial until he was almost dying and the Mikes found him".