Did you try adding these things? Because if you create a prompt on your hierarchy and then in the analysis you add the hierarchy and say it is prompted it works.
Don't you have a possible conflict in what you ask for? A prompt on time hierarchy but filter on last 6 months of data? Why don't you filter directly on the last 6 months of data then?
Or do you mean to let the user select a month and then your analysis must filter on that month and the previous 6 months? So do you really want a time hierarchy prompt as you have a 6 months window on your data? Isn't a month prompt more adapted to the needs? (you can still have a time hierarchy in your analysis, but I just don't see it as a filter).