You're right, Excel does save back to CSV. It discards half your changes because they can't be saved in a CSV, modifies anything that could possibly be a date, and rounds random numbers, THEN it saves it as a CSV.

Before discarding non-compatible changes, it'll tell you, and offer to save as XLSX.

If you're just editing some values, you can save back to CSV with CTRL+S no problems.

No, you can't. It will import and re-export every cell, not just the one you modify. This includes interpreting dates and rounding numbers that exceed Excel's max value (instead of interpreting as a string)

They have a small banner at the top to warn about this, but it doesn't even address the things that were already lost the instant you opened it.