I'm surprised that your landlord or their agent scheduled walk throughs while you're at the property. Is your landlord present, or do people just show up to do a walk through? If your landlord or their agent are present, and you're talking shit about them and the property to prospective tenants, you're a legend.

Everything should be in the contract before you sign it. Cost of roof materials, cost of labor, cost of wood replacement if needed, cost of joist replacement if needed, etc.

And yes, you should definitely take them to court for not using the materials specified in the contract and get them to replace the materials with the material specified in the contract. Plywood is more expensive than OSB, and they charged you for plywood and then used a cheaper material.

You can build extras into a contract before you tear off a roof. Like, if joists need to be replaced, this much per joist, wood replacement, this much per sheet of ply that needs to be replaced, etc. Never sign a contract with an open clause.