I've been in this role for the last 8 years, so I think I can offer some insight. Responding to your questions in order:
I don't think the variation in pay are closely related to companies. I would say industry variation is much more of a factor. I know glass manufacturing has much higher scales than say plastics.
Top earners can make as much as $120-150K including incentives. I would say $80-110K is normal.
This role is all about improving manufacturing process yields and managing teams to achieve results. Lean manufacturing principles, six Sigma, MBA's, and other process focused credentials are the buzz words of today. Bottom line is that a good manufacturing manager must be able to evaluate process weakness, develop an improvement plan, and implement that plan with measurable financial impact.
Hope this helps...