No data in excel is safe.
I can easily bypass / remove / reveal any passwords in the workbook, worksheet, or VBA level.
(for less than 100$ -last time i checked- a person can buy a commercial application that does the same thing)
If you're trying to restrict access permissions to a portion of the data, try separating the data into a secure and insecure workbooks and keeping the secure workbook in a location that only certain users can access.
And I dont think that using encryption features unique to a later version of office would make it backwords compatible... If this is something that you need to use across yoru cases, then this is something you can easily test for.. (by trying it out)