There is no way to add a password-to-open to the spreadsheet. Ruediger offered to custom develop this, but I came up with a workaround.
Ease of use
Very clear to use - I barely needed the manual.
Unusually outstanding support - A+. Ruediger responded immediately to a minor issue I had, and quickly developed a patch.
Very good, although not really needed for most use cases once you understand the basics.