Sorry to hear about your troubles. Sometimes there are problems with getting Award flights ticketed with Star Alliance partners and you've seem to run into one of these. Your mom should enjoy the trip on EVA in Premium Economy just fine; not as posh as business, but it'll be a comfortable ride and a memorable one, especially if she likes Hello Kitty.
Regarding the $50 that you're looking to have refunded, if you are at a dead end with United perhaps you can call your credit card company and explain the issue to them? They might be able to refund the fee.
I've learned that when speaking with any customer service representatives, regardless of them being employed by an airline, insurance company, product support, etc., it's always best to use non-threatening, non-attacking words when speaking. This usually will get the CSR to want to spend time talking with you and helping to resolve your issue. It's not always easy to maintain the type of emotional state when something bad or unexpected or undesired happens, but if you do you might just get your desired outcome. As the expression goes, it's easier to catch flies with honey than vinegar.