Please read the forum etiquette. Posting multiple times and expecting other people to help within the minute is not the way to go.

You probably don't need a "union" of different datasets. In my opinion, you could use expressions to build the MDX statement up dynamically and based on the user selection.

If you want any further assistance, you'll need to try something first and post your attempts with examples of your structure. Without that you are unlikely to get any more help.