I might suggest that you try to keep one topic per post..
With regards to question 1: The current approver should be getting a reminder to approve email every 24 hours, so I'd expect that they are still getting notified. You could always go to site management -> logs-> Email Log and resent the original approval email that was sent to the approver.. I'm not sure what you are looking to do when you say "can the workflow be triggered again". Is it simply the approval notification you are trying to resend? You can always adjust the approval process back by opening the request under My Requests, and going to the approval status tab for the request.
With regards to question 2: You can add conditions to your upgrade campaign... This allows you to include/exclude specific users/devices. There is no user interaction in either a retire or upgrade campaign, beyond an optional notification to the user. If the software is installed, and if the condition evaluates to true, then the software will be retired or upgraded accordingly.
With regards to question 3: I'm not sure that I completely understand what you are looking to do here, but I think that your options are going to be limited.. Probably your only option would be to create some type of condition.. For example, an AD group which contained the allocated users, or perhaps an collection.. Not sure if you would be able to maintain this collection/group dynamically, or if you would manually have to maintain the member ship.
