On Get on Board you can reopen jobs that you have unpublished and that are less than 4 months old. If it's older than 4 months, you won't be able to reopen it
Yes you can. If you have locked jobs that you wish to unlock under your subscription plan, you can use the button "Add this job to your plan". This button will unlock your job and will make it part of your active jobs quota, just as if you had created the job after subscribing.
When you unpublish a process, it stays open, so it continues to appear as an ongoing process on the company's dashboard and on the dashboard of the professionals who applied for the job.
If you have a Get on Board subscription plan, you can access Talent Database, from where you can invite the professionals that interest you directly to your open selection processes.