I cannot answer your question, but I'm really curious how Alaska deals with people who are relocating to Taiwan (for work, school, or retirement).

You might see if you can get free access to Timatic to determine what requirements Taiwan has for onward and/or return tickets for visitors. Timatic is what the airlines rely on to determine whether passengers meet immigration requirements at a destination airport. It looks like UA provides free public access to Timatic here:

https://www.united.com/en/us/travel/...l-requirements

Did Alaska happen to mention who, exactly, is going to find you if you cancel the return ticket it forced you to buy? If this is just a scare tactic that the Alaska agent used to get you to buy another ticket, then a complaint to the U.S. DOT would be in order.

By the way, are you certain that you were actually talking with Alaska, and not a scam travel agency?