A couple of thoughts....

The Wal-Mart engineer was not an "approved" signer, so the change order was illegal and not recognized by Wal-Mart.....so I am assuming that you did get a change order before initiating the work...

You had a very bad lawyer, who chose to give up(after all, it was your money, not his, on the line). He folded your case and your cards when confronted by what you inferred was a ton of legal opposition... Just a thought.....