What do you want your main dish to be? Casserole? Sandwich? Heavy finger foods? Main dish salad? Dairy and nut free would be my main concern.

Also, hot or cold? Fancy or everyday? My first thoight was something in the picnic family, being July, like tuna salad/salmon salad/shrimp salad, depending on your druthers. With sandwich fixings, side salads, etc., inc. leaves for keto wraps. Similar kind of thing for hot with cultets of chicken, fish, etc., or sausages of the same. Or for fancy, if at least one of the kitchen helpers knows how to carve, you could set up a carving station with roast beef, chicken, and or a whole salmon or similar large fish.

IMO, when feeding that kind of crowd, simple is best, steer the special diet needers to what's best for them first, and have enough variety that everybody can make themselves happy.

A comprehensive Make-Your-Own Sandwich bar could offer lots of choices for various diets and ages. (Many types of breads, rolls, leaves for wraps, meats, cheeses, sliced veggies, spreads.) Since you have AC, you could also offer a cold-soup bar (gazpacho, cucumber, and some pretty fruit soups). Given your kitchen staff, you could also plan a succession of fresh-baked variety pizzas for many different diets.

For guests tempted by unhealty snacks, one of the most well-received parties here was a huge collection of Chips and Dips, (and I do mean Huge selection); people had fun trying all the unusual tastes, comparing notes, and making suggestions to one-another.

Without knowing much about the purpose of the event and a bit more about the attendees, I’d be inclined to offer mostly cold foods: grilled chicken salad with balsamic vinaigrette, maybe some premade roast beef or ham sandwiches with condiments on the side, mixed bean or other legume salad, potato or pasta salad, tomato cucumber salad, cubed watermelon-cantaloupe-pineapple, your favorite chips. lemonade, iced tea, water ( with mint sprigs or lemon slices to fancy it up if desired).

Or if you want to go fancier, another thread gave me a thought, if your kitchen helpers are actually good with fiddly, you could do spring rolls. They make keto wrappers, and I've seen recipes that use whole cabbage leaves, if it wouldn't bother the keto eater(s) to have a separate version.

Filling out the sides is pretty easy once we know the kind of main you want.

To answer your questions and add a bit more detail...It is a family (all relatives) getting together to celebrate an older adult big birthday and visit-people from different areas of the country. It will be very informal and nothing fancy or fiddly, just good food. Not needing to impress anyone. We are all close and comfortable in our own skin. I am not wanting to plan the main dish for the special diets, but would like to be sure they could make a meal out of the offerings. The special needs people are a son, grandson and granddaughter, and they would be the last people who would want a big deal made of their diets. I just want to consider them, but if necessary will have one offs in the kitchen for them. I do not want a fish/seafood main dish. I believe the pescatarian can eat the same food as a vegetarian, plus fish. The GS who can't tolerate meat loves cheeses and hummus and before I finalize the menu, I will check to make sure something works for each of them.

Good info. Yes, pescatarian can eat vegan for sure, most will do lacto-ovo, but some eschew all land based animal products.

So the question is whether you want interesting or comfort tradional. For the former, perhaps a Mediterranean mezze kind of spread, with hummus, baba ganouj, carrot salad, cucumber tomato & onion salad, a spicy pepper salad, a not spicy, crunchy peppers salad, a bean salad, etc. You could add chicken and seitan kebobs. Pita and big leaves for wrappers.

Or an ersatz weenie roast, with a variety of sausages, including vegan, chicken and meat. Rolls, leaves and fixings, and low sugar baked beans, cole slaw (or broccoli slaw), potato or sweet potato salad, cauliflower and shaved Brussels sprout salad, etc.

Maybe add a couple of stand up nibbley foods if guests will mingle prior to lunch. Nuts, deviled eggs, some kind of a bite-sized tartlet, cheese straw, spanakopita, or bacon wrapped date kind of thing. I would not bother w/ veg & dip.

What do you have available for service? If you have outlets and hotpots, you could do things like baked beans, hot meatballs, sausage and peppers, hot pilaf or other rice dish, pasta dishes, pulled meats or jackfruit. Taco bar is often popular. If you can do ice bars, you can do all sorts of cold stuff. Salads of various kinds, sandwich bar fixings or other breads and spreads, cheese assortments, veggie and fruit assortments, ect. Deviled eggs and pickled things are often good.

We used to have a big family reunion on Labor Day weekend every year. We kept the menu pretty simple with barbequed brisket and chicken, potato salad, baked beans (vegan), pasta salad, fresh fruit salad and an assortment of breads and condiments. That menu usually provided something for everyone to eat.

For that kind of meal, I go plant based with some meat dishes for those who want it, - simple roast chicken, succulent roast leg of lamb - feeds lots of people for minimum effort! Bready things for those who can eat it.... does everyone eat fish? A cold, dressed salmon can be a thing of beauty. I don't know anyone who doesn't wolf down Turkish veggie dishes.

https://www.dailysabah.com/turkey/expat-corner/mad-for-meze-the-definitive-guide-to-turkeys-starter

In my extended family, for a gathering like that we would probably do a baked ham or two, some baked or grilled chicken pieces, sausage w/ peppers and onions, and some nice dinner/sandwich rolls (& appropriate condiments to make sandwiches). Then huge pans of sides like: creamed potatoes (or scalloped potatoes or potato salad), ziti and sauce, mac & cheese, coleslaw, green beans, garden salad, fruit salad, etc.

eta: mac & cheese isn't a thing around here, at least for me and the people I know, but it seems quite popular with a lot of people in the ether, and you had mentioned it, so I listed it. :)

I would do grilled chicken (make sure it is not seasoned, marinated, or sauced/glazed with anything sweet, therefore negating the keto option) , a big pan of roasted, non starchy veggies, a big salad bar with lots of topping options, and a baked potato bar with even MORE topping options. Add some dips like hummus, guacamole, salsa, and things to dip into them and a cheese tray. I feel that given enough options of toppings on the salad/ baked potato bar, anyone can make a meal out of either.

IMO, Mac and Cheese goes with everything.