You should really rephrase this comment as "WN doesn't have a ticket agreement with UA," as it is their business model not to engage in interline agreements that drive up their costs.

As unfortunate as your inconvenience was, I think UA has done all it is obliged to do. Once you decided to seek your own travel alternatives, you are on your own. Refunding the unused portion and a $200 cert are adequate compensation, IMO. I would still write UA an email, but I would be doing so with an expectation of an additional apology and nothing else.