MANN+HUMMEL: Organisation & Management
Apparently the ultimate parent company of WIX filters is Mann-Hummel, a germany based company. (link to their website above)
They also make/own Mann and Purolator filters among others...
Since this is a large multinational corporation I really doubt that getting a lawyer involved will result in getting any assistance in paying for a rebuild. Once an attorney is involved, it goes over to their legal department, and you will become just a small bug to be squashed.
My suggestion is preparing your written statement of what happened, what the conversation was with the district mgr. regarding the missing bypass valve, photos of the filter, and a written estimate of the cost of the rebuild. Send to the district manager and ask him to elevate this to his manager. No threats at this point - just politely request their assistance in making this right.
The issue of the "wrong" filter being used is a moot point as their own engineers will know that either filter will work fine with this engine.
If they are as concerned about quality and customer satisfaction as their websites suggest, they may be willing to at least help with the cost of the rebuild.
If they refuse, then let them know jut how far your are willing to spread the word on every available media so that others can be warned as to how they choose to treat their customers who have purchased their defective products.......