A- I would have put my foot down on the inviation list. Your wedding has nothing to do w/ your baby shower.
B- as such, if you're doing "regrets only" and you don't even know these women's' names- I would imagine they are getting the invitation and wondering "WHO?" and thinking "I'm not bothering to reply to this".
Which leads to:
C- yes, if you did "regrets only", there is a good chance you'll have a lot of extra food. People are horrible about RSVPing in general, and regrets only seems to make it easier for people to not do it. I've been to one "regrets only" party and they had SO MUCH extra food.