Is there a way to get a list of all questions that potentially give a necromancer badge?
That is, other than going through the old unanswered questions and manually looking for ones with 5 or more upvotes.
That's not how the necromancer badge works. If you answer a question more than 60 days old, and your answer gets 5 upvotes, you get the badge. The question doesn't have to have any votes at all.
So to find questions that you can potentially get the badge from, just sort by newest and go back enough pages to find two-month-old questions.
Perhaps the description could be improved. "Answered a question more than 60 days later with at least 5 votes" does make it a little ambiguous as to whether the question or the answer must have 5 votes. Maybe "Answered a question more than 60 days later and earned at least 5 votes"?