Well at least two different plumbers have come up with the same diagnosis so I don't think anyone is trying to swindle you into doing something for nothing. You should have the floor broken up and ALL of the bad piping replaced or you are just going to chase this problem for years to come. As far as cost, get three estimates and go with the middle guy. In all good faith I don't think it would be fair for anyone to shoot a price at you because every job is different. You will need to call your insurance company for the particulars. As always the coverage depends on your policy.