It looks like there are some good answers gere, but I'll throw in my two cents.
When it comes to flooding in the basement (or any other issue) the seller is disclosing issues they are aware of. My grandfather passed away in 1979, and after that my grandmother never went done the basement. When she sold the home there could have been four feet of standing water down there and it wouldn't have been on the disclosure because she wouldn't have known. The property having been used as a rental has no bearing on the present value or use and I don't think that's even on the disclosure form. For the pool, if you can get the name of the company that closed it down, they may guarantee their work, but don't count on it.
Your agent may have represented you if you signed a repesentation contract, and he may have represented the seller as well. But his expertise lies in the buying and selling of real estate, so he may not be an expert in waterproofing, rental history, and pool care.
I would hope you hired a home inspector before making this purchase, and if the inspector missed the evidence of flooding, or problems with the pool plumbing, you may have recourse with the inspector.
1) The tough part is proving that the seller INTENTIONALLY lied to you. They are protected in a sense, because sellers are obligated to fill out the form to the best of their knowledge.
2) I agree on the flooding...even if it was fixed properly the seller should disclose this to you. I am not sure about the rental part and why that would be a material fact, but I have never had that one pop up yet.
3) As for the pool are you saying that the seller told you they used a company and lied? Or, that they used a company and they just did a poor job? Why was the pool not properly inspected during your contingency? They are money pits for Minnesota yuck.
Dual agency is brutal for these matters. 9 out of 10 times we usually sign a buyer rep agreement to at least give you some protection if the seller agrees. Most of the time there are no problems unless it comes to this. Regardless the seller always has to disclose everything to the best of their knowledge period.
If you signed "the arbitration form" as part of the PA you elected to use that route instead of small claims court. To start that process is fairly easy, but I agree with everyone best to get a real estate attorney (i can not stress enough the "real estate" part of it) to review the docs for you.