You are wrong. In part1 it was stated only a Sannin can defeat a Sannin. And this was true until Sasuke defeated Orochimaru.It was never stated that they were on par.
I don't think Orochimaru had Edo Tensei before he left the village.
I don't really think it's actually that Slug>Snake>toad thin, but I may be wrong.
There is a Japanase myth and card game, where the Slug is afraid of the Toad, the Toad is afraid of the Snake and the Snake is afraid of the Slug. And we all know Naruto is based on japanase legends and myths.