A). Create criteria-based opportunity sharing rules for each product line. This allows you to specify criteria for sharing an Opportunity with a specific user or group of users. For example, you could create a rule that shares an Opportunity with the product engineer when the Opportunity has a certain product line assigned to it.
B). Enable Default Opportunity Teams for the Opportunity. This allows you to set a default Opportunity Team for an Opportunity, which will automatically assign the product engineer to the Opportunity when it is created. This is useful when you need to assign a specific user to multiple Opportunities.

Marketing Users can import leads and use the campaign import wizards if they also have the Marketing User profile (or the "Import Leads" permission and "Edit" on campaigns).