"They have transformed the nation that was once known as the leader of the free world into a crime-ridden, open-borders free-for-all where opponents of the regime are arrested on bogus charges. "
I-864 is not needed for a K-1 visa. I suggest you study this guide:
K1 Fiance Visa Step-by-Step Guide on Filing an I-129F for a Foreign Fiance - US Immigration & Visa Guides - VisaJourney
Not sure what you mean by that, but marriage to you confers no immigration right, nor does an I-130. Once an I-485 is filed, she cannot leave the US without abandoning her I-485...unless she has an approved advance parole document in hand.
Unless your employment will continue after you re-locate to the US, an employment letter is useless and not needed. You can claim only income which will continue after re-locating. Unless you can show sufficient assets or sufficient current income which will continue, you will need a joint sponsor.
LONG LIVE TAIWAN!!!!!!
That is another issue. Why was the immigrant visa denied?? There seem to be a lot of moving parts in this case. Regardless, all that seems to be water under the bridge now.
Bad advice. She cannot enter the US as a visitor with the intent to stay and adjust status. It sounds like you already know that.
My advice is to marry online, meet, then start the spousal visa process.
But if the extension was denied, then she could have been assessed with an overstay. If so, then that is , likely, the reason for the ESTA denial and the B2 denial.