With some error, I cannot see your whole dashboard, but anyways,
You can put the field for display and field for sorting separately.
Below field can be used only for sorting purpose, always keep ascending , but with "-1", that particular case becomes descending.
Have attached workbook with some workaround (version10.2). Hope it will help you.
I tried workaround for your first two questions, whereas for third question, as of now there is no way to edit/customize the sort button on tableau.
Thank you, Shin. I got this to work. I appreciate you attaching the workbook and providing the screenshots. Much appreciated.