1. Sasori won't be able to locate Mei in the mist so will not be able to pinpoint her position, so will have to randomly fire unless he enters the mist or Mei leaves the mist both cases not happening as Acidic Mist will melt him.Alright I read the whole thing, and even though I think it's ridiculous to restrict poison, since Sasori's whole fighting style relies on it, I'll counter your points.
1
Sasori's Iron Sand breaks the sound barrier [You must be registered for see links], Mei will not be able to dodge them, there's nothing she can do fast enough to block them either.
2
Since poison is restricted, I take it a poison cloud [You must be registered for see links] will also be restricted? If not, then this is a very viable counter to the hidden mist tech, and Mei will have to hold her breath, meaning she can't perform any techniques that come from her mouth.
3
How does Mei counter this technique? [You must be registered for see links] If Sasori is surrounded by mist, this technique will spread in all directions and Mei certainly can't dodge this since it goes in completely random directions.
Your points would work if Sasori stood there doing nothing at all, he's always been shown to dominate the battle and pressure his opponents.
2. Any form of poison is restricted. And i do agree that with poison, Mei doesn't stand a chance.
3. Firstly, Iron dissolves in water and much faster at acidic conditions which prove it useless. Also, a Water Dragon capable of pushing a Susanoo back should be able to push that back. Secondly, Acidic Mist melts his chakra strings as Icelerate has said.
I have said it won't be easy for Mei, and I know Sasori is a strong opponent but i believe Mei wins 6/7 times out of ten. Thanks for reading though.