While you're mentioning single-byte assumptions, add some more: - All space, dash and underscore characters are single-byte

I might disagree with some on your list, such as the below: - Excel can losslessly save CSVs it opens

This is not always false, depending on how careful the user is. Double-clicking to open a CSV file is definitely, but using the menu to import data, then going through the UI to treat every column as Text (and not General) should be fine

I might also add a couple: - A header row, if present, only spans a single row