If possible please supply a workbook so that others can see your work and the specific issue. A lot of the answers provided are dependent on seeing a bit of the data involved.
For the first item - when dealing with a top x and trying to get a total for them, I would try to get those values to appear with a calculation that only returns the values for the selected top n values and then create a different view that just has the totals. See the attached example workbook with the dashboard that shows a dyanamic top N with grand total that adjusts based on the selection of top N. I utilized a horizontal layout container to make the totals move up/down wit the selected number of Top n.
For the second item - does the dashboard provided get you what you are looking for? I need a bit more details here to help.
I hope this helps.