I've been a groom's mother. Please listen carefully. You are right in everything you say. However, don't get your fiance into a discussion with your mother over this. It's YOUR responsibility to talk to your parents privately.

Ask them for a time when the three of you can sit down together and discuss things you need to talk to them about regarding the wedding. If home is more comfortable, that's awesome. Simply give them an update on the current plans, etc. In the middle of this "update" tell them that your fiance's parents, she and you have decided that you want the wedding to be at this particular venue. If (or when) your mother starts worrying, LISTEN to her. Acknowledge her concerns. ("That makes sense. I see what you mean. hum.......... let's see........I have an idea. What if the three of us visit the venue together and look around to see what it's like. I can call and make an appointment.") When she continues to tell you things that make you realize she's worried about being in an uncomfortable place, just assure her that it will be wonderful. Explain that everything will be taken care of so that the parents will have special seating, etc. She needs to understand that someone will be telling parents what to do, and where to sit. She won't have to make any decisions on how to behave. If you can get her to understand this without being condesending, she'll be on your side. Give her some time and space to think. I think she'll come around. Meanwhile, go ahead and book the venue. The bride's parents want it and they're footing the bill. It's their call. Just be gentle with your mom. It'll be okay!