i have a pretty thorough pre-made presentation to do with bands when i first meet them that explains the differences and identifies important terms so there are no misconceptions. i mean that's the way i do things but it may work for you if you really explain to them what the differences are so there is no confusion. when working with a more unestablished band you always hope that band makes it big because it's a good notch on the belt to be part of something at its genesis or early stages. realistically speaking i always see it as there has to be ONE person somewhere being a producer. choosing vocal takes, etc. that is all on the job of someone who has the final product in mind. i always viewed an engineer as someone who just takes care of technical stuff.
i've worked with bands where one person in the band has been the producer and he has been there for most of all the sessions and i've just engineered and they have said ok that's the take or sing it this way, that felt too fast, etc. etc.
the X figure that says those things and "calls the shots" or helps guide songs is the producer and more often than not bands will grant me the title of producer or sometimes shared co-producer with an individual in the band if not the whole band.
just the way i see it at least, hope it helps.