Ooh, good luck with this one! We had some nay sayers at my wedding as well, but here's the thing - they are going to go - have the absolute time of their lives and be thanking you after it's over.
In regards to the invite issue - tell your MIL and SIL that if they want extra invites to go out - they need to get off their butts and help put them together. Maybe that will shut them up!
The other thing to consider is, no matter if you're doing the wedding at home or a DW, people are going to complain. Weddings seem to drum up stupid baggage and when you're dealing with someone elses family, it just doubles the trouble!