I would not make any default determination until and unless there was a proactive user action. If there is any value that doesn't round trip through string serialization, don't allow it to be coerced without the user deciding to allow it, explicitly.
No, but I think it's a lot. I'd bet that the vast majority of those round-trip through string serialization, requiring no further user action based on my recommendation.
Disagree, the most common way for an experienced user to type a date into Excel is as a partial date if it is this year, eg 4-Apr, which is instantly converted to 04/04/2025 or whatever.
The vast majority of dates I personally type into Excel would be in this form.
How would you, if you were programming excel, determine whether the 5-digit number entered with a leading 0 is meant to be a zip code or not?