Lots going on there, but I have to ask:
You're working day to day, files coming in all the time. Some tupperware lady comes in and wants some signage, provides you files (raster, vector, who cares...).
What do YOU do as a professional to reasonably research the lawfulness of you printing this customers stuff?
Your example about a Disney character... I don't know what a Disney/Marvel/Lucasfilm character looks like. I don't watch cartoons or superhero movies, I'm busy trying to work. How am I supposed to know what is legal to print?
I would say all quotes/invoices should say "customer owns all supplied artwork... etc..etc.."