Not necessarily - they could have asked someone else to swap seats. Likely a male, and possible a UA staff member if there was one in the cabin (or pilot rest maybe?). No free seats in business on HKG-SFO would certainly not at all be surprising.

Based on the description in the article (which is obviously not the entire story) it's hard to see what UA could have done to avoid things. Yes, the original FA should have acted differently, but presuming she spoke to the purser almost immediately after that FA then it almost certainly didn't change the outcome in any way.

Serving the passenger "at least three to four vodka sodas" over potentially a period of several hours is hardly what I'd consider "excessive alcohol".