First off, it's great that you are being so conscientious here! A huge number of people don't do their homework. Some get grumpy about making accommodations, or snark about it not being "real food"/insist that it would be better if they'd been able to include the offending ingredient.
Secondly, I think it's important that you realize that very few hosts are as mindful as you are. As a vegetarian, you can probably relate to people not quite understanding the restriction. I'm in the habit of double-checking on things like soups and sauces because most people don't even think about chicken or beef stock, let alone anchovies. (I've even had people try to tell me that a dish containing processed pork was vegetarian... as in an actual argument.) Don't be offended that someone is trying to do their due diligence about what they put into their body. It's taking personal responsibility, which is exactly what they should do.
Unless you're a flexitarian, you probably do a bit of recon when you eat out or at an unfamiliar person's house and want to check in on hidden ingredients. It's not that you think they are incompetent; it's that you know that they may not be completely familiar with your restriction, or have a different understanding of it than yours. (If you look around on this board a bit, you'll see people suggesting fish--or sometimes chicken--as a vegetarian option. My definition of vegetarian excludes all animal flesh and all slaughter products.) Heck, sometimes, it's just autopilot that turns the vegetarian risotto into a bacon-y no-go.
I think the truth lies (as it often does) in the middle path.
Mr. K is correct that you should give Marge more information than just the names of the dishes.
You are correct that you needn't account for every teaspoon of oil or cup of lettuce.
Send word with the Mr. to Marge. Use TootsNYC's and NyaChan's advice. Let her know what the meal is and how you are going to accommodate her restrictions.
"Marge, we're looking forward to having you. As Homer may have told you, I am also an ovo-lacto vegetarian. As I understand, you have a gluten sensitivity..." (this tells her that you are aware of her restrictions and also gives her a cue to give you more information if necessary) "I'm planning to make a vegetable alfredo lasagna with No-Gluten Brand rice noodles and GF-certified alfredo sauce from Local Co-op. I made it for my cousin, who has Celiac disease, last year, and it turned out very well."
"The salad will just be a simple green salad. I'll have a few dressings. We can read labels before dinner. For dessert, I'm making an apple crisp. I was going to use the same recipe I used with my cousin. We used Paddy's Rice Flour for the crumb top."
That will give her the peace of mind and the opportunity to let you know if the status of any of the products has changed. (Like the quesadilla stand finally using rennet-free cheese.)