Posting a packaged workbook will always get more responses here, but if I understand your description right, you can use a simple table calculation to only show the latest month.
For your yearly rollup, keep your month field in the Rows or Columns, so that you are still displaying all months of the year. Then create a calculated field called "Last" with the following formula:
LAST() = 0
This will return True only for the last row in your table calculation.
Add this calculated field to filters & set it to True. Make sure addressing is set to your month field, and you should only see that month now.
Then uncheck "Show Header" by clicking the little down arrow on your month field, so that you aren't displaying the month anymore (you can also format the month to show just the year ("yyyy") if you want to display the year.
Hope this helps - if not, please post a workbook.