How did Satan get into Eden?

There is general agreement that Satan had already rebelled when Eden was created and that Eve sinned just a few hours/days after creation. Are there any hypotheses about how satan got into Eden immediately after its creation? It's rather unbelievable that he was admitted there after he had been banned from Heaven. Secondary questions with no answers are: how could he take the shape of a serpent, speak Eve's language, convince her etc.