1) by clearing the session on page load, each time you load the page it is performing the sort for the first time, you will need to use a reset of some sort to trigger clearing the session you sort ask how to do the reset in your 2nd question
2) set the link to have a UR variable:
page?clear=true
go to the bindings panel, click the plus button and select URL variable. name the URL variable "clear". in the clear session value behavior, click the lightning bolt next to the trigger and select the clear url variable binding. This way the session will only clear when the link is used