not quite sure about that answer about which timeline is the alpha...i thought that when someone changed the past the changed timeline becomes the alpha and the one they come from is doomed (hence "dead daves piling up"), other timelines dont pop out of nowhere...?

The changed timeline is always the alpha timeline. Doomed timelines exist to maintain the alpha timeline. That person was always meant to travel back and change that thing and alter what would have been their timeline into the alpha one.