As far as explaining why one can't just immediately suicide as soon as they enter limbo:
I don't think anyone has touched on this, but I would assume when you wake up in limbo alone, you are already unconsciously populating the world with your shit, and it feels like real life to you. You go on living your life until either you die naturally or something abnormal happens to make you realize you are dreaming.
I'm not sure why you can't use your totem though.