The US do not need to be in the strait to protect Taiwan from an invasion.
Outside of the winter months, China have at best a %20 chance window of an invasion. Since I mentioned the winter months, it implied weather as
THE dominant factor, and since weather is neutral, as in no one controls the weather, this mean both China and Taiwan know at least down to the week, if not the day, of when an invasion is feasible inside that narrow %20 window. This mean the element of surprise was never there, and when you cannot surprise your opponent, the difficulty of combat doubled and correspondingly the odds of victory drops by 1/2.
In what ways does surprise matter so much? If your opponent is asleep, for example. If he was asleep, his senses are dulled, his reaction capability diminished, and most likely he is unarmed. Another example is the Battle of Midway when US fighters caught the Japanese while they were changing ordnance from torpedoes to bombs.
For China, there is no element of surprise for an invasion of Taiwan, and if Taiwan can prevent an invasion, Taiwan win. But prevention is not the worst. If China launch the invasion fleet and somehow, with or without US help, Taiwan inflicted so much damages on the invasion fleet that China have to abort, that
WILL be seen as a victory over the PLA and it will reverb all over the world. The abort will be both a military defeat and a huge loss of face for China. Sympathy will pour in for Taiwan and the odds of independence could cross the %50 threshold.
I doubt you have any idea of how difficult is that -- 'not that easy'.