I hope she does not visit, and I hope China does not shoot down her plane if she does. Honestly, it will be best for everyone involved if she just died of natural causes on the way.
The PLA can warn them all they like and fire warning shots, sure.
But if they disobey the order -- or alternatively, if the civilian airline aircraft (likely to be govt chartered aircraft) is escorted by some US fighter aircraft on the way into Taiwan's de facto airspace -- would the PLA still actually shoot down the aircraft?
It's a fairly simple yes no question of "will the PLA actually shoot it down".
Personally I think the answer is no.
Why send fighter jets to intercept the plane if the Chinese government is not willing to go all the way? The PRC government does have a valid excuse to shoot down the plane: it is entering Chinese sovereign airspace without permission. That is an act of war.
Shooting down anyone's plane during peacetime is one of the worst ideas anyone could've come up with.
Not just that, we are talking about shooting down a plane carrying someone who would be taking the leadership helm of the United States of America in case the President and Vice President got incapacitated and unable to perform their duties.
There is always the diplomatic table that everyone can return to whenever situations got intensely worse. Opening fire and downing some VVVIP of your arch rival is always the last ever option that should only be explored when no other alternatives are present.
Remember that even during the absolute heights of the Cold War, neither sides dared to come up with ideas of assassinating high-ranking government officials of the opposite side, even when both sides are heavily involved in proxy wars, staredowns and standoffs right at each other's throats.
In fact, China still has tons of cards on her hands to play with, especially regarding the industry, economy, trade and finance. It is the US and her allies who are throwing away more cards than they realise.
It is a Mexican standoff unfortunately
Mexican standoff will remain as Mexican standoff as long as nobody pulls the trigger.
When the trigger is pulled, all bets on a peaceful resolution is out. The only result would be bloodbath, until either side got decimated, or both sides got exhausted/decimated.