John, couple of short answers:
- When you run into the agg/non-agg problem, try using ATTR() around the fields that need to be aggregated
- Also try taking out all the aggs and then letting them get aggregated when you put them on a shelf
- It sounds like Action Filters would be the easiest way to drive your dashboards. They can act as a global filter, or you can set them up to effect selected sheets and selected fields.
The agg/non-agg thing can usually be sorted out. But in this case I think Action Filters will be easier and quicker.
Here is a KB article to resolving the error:
Using the action filters as Shawn described might be a good solution!