Feeding 8 awkward guests.......

How about marinating several butterflied chickens with garlic, salt, some herbs, olive oil and lemon the day before and roasting them on the day of your party? They take less time to roast than regular chickens. You can also roast some cubed potatoes alongside in the oven for a side dish. While the chicken and potatoes are cooking, you can assemble a salad too.

As a side dish, panzanella keeps really well overnight. You can just leave out the onion.
https://food52.com/blog/10820-how-to-make-panzanella-without-a-recipe

Caprese salad is my go-to entertaining dish this time of year. Serve it with chicken kebabs (you can marinate & assemble in advance, cooking won't take long at all) and some kind of whole-grain or pasta salad that you can make ahead and serve at room temperature. The best part is all these things are easily scalable and make great leftovers.

You are already a wonderful hostess for asking! After realizing the full implication of no onion (hopefully that isn't shorthand for no alliums, but either way: OOF), I'd go for a Middle Eastern/Mediterranean inspired meal. Lots of fresh meze which are all perfect for day-before-prep, cool salads, and then simple kebabs (these are my go-to simple kebabs http://www.annies-eats.com/2010/05/11/garlic-lemon-chicken-kabobs/), warm home-made pitas, and baklava from your fave Mid/Med bakery. It'll be a great, social way to enjoy a flavorful all-those-things-free meal together. Good luck!

I would do a huge anitpasto platter with cheeses, marinated veggies, olives, grapes and foccacia on the side. Then on to several do-ahead composed salads, with protein (I think you could have at least chicken, hard cooked eggs, smoked or grilled salmon) served on the side. Think Ottolenghi's "Plenty" with veggies as mains, proteins as optional add-ins. Or a soup bar (a few hot, one cold) with salads. All can be done ahead, most of the time is spent arranging.

Chicken Marbella from Silver Palate cookbook. I'm pretty sure it doesn't contain any of the forbidden ingredients you listed, and it tastes even better when you make it a day or two ahead and reheat it. If your guests don't like this dish, then nothing will please them!

As for sides, I'd suggest googling "side dishes for chicken Marbella" - this recipe has a loyal following that I'm sure has spawned many side dish conversations.