You originally mentioned that when it popped the dome light would dim & I wonder if that is from the starter partly or wholly engageing at that point in time (which takes alot of current & will make a light dim). Could you take off the brown (sol) wire at the starter relay in case the relay is intermittently engaging the starter then jump it at the starter to start it & retry it & if it still does it then replace the starter to elim that issue. I'm bothered by the 1 spark you saw & was a helper loading the eng as you eyeballed things under the hood?