I'd talk to an allergist.

I have not heard of DMF being sprayed on furniture. Its actually small packets that are placed inside and around the frame and usually limited to leather furniture imported from China. To prevent mold growth in transport. Back in 2010 the EU banned such use in upholstery. I don't think the EPA has banned its use as of now.

You didn't mention what type of material; fabric or leather, you could have the material tested by a toxicologist. I doubt you will get much help from Williams-Sonoma. To file a claim you would need to provide direct evidence of a substance; cause & effect.

Thanks, this gives me ideas

Do you ever have the symptoms when you're not sitting on the couch? Have you tried covering the couch with a blanket and sitting on it that way, to see if you still get the rash?

It's possible to develop hives from pressure on the skin -- look up dermatographia. I have chronic hives and when they are really flaring up the pressure from sitting or lying down will cause a very itchy rash.

I would try covering your couch with a cotton blanket for a couple weeks, using couch like normal, and seeing if you have the same reaction.