Mainland China is about 100 miles away. Thats a damn long distance. Strictly from an engineering perspective, that would be incredibly impressive feat if they did it.
Did a quick search and it looks like the longest rail bridges now hit a max of roughly 1/4 what this Taiwan-to-China bridge would need to be.
True. But this would not be a cheap project if they legitimately attempted it, so they probably would want to keep the Chinesium quality to a minimum so it doesn't fall apart after it's first use.
Chinesium lol, even so, even if they get the best quality everything and actually make it structurally sound. It's the Chinese, they'll fuck it up somehow and blame it on western sabotage.