Can I write a liability waiver into a written agreement for a local farmer who wants to store farm equipment in my barn

You can certainly write in a waiver but I would have the local farmer get renter's insurance on the equipment he stores in your barn.

My first question back is whether the company provided anything in writing regarding the ownership grant (e.g., stock purchase agreement). If they...

Breach of Contract, our supplier did not uphold terms on his invoice.

If they have not delivered in accordance with the PO (is there any additional agreement(s) in place for this transaction) you would certainly have...