Can the olives be substituted or left out?

Another idea - use a vegetable pickle (giardiniara, radish, onion, sauerkraut or kimchi) to give the dish umami and sour tastes similar to what the olives would have given.

My husband doesn't eat olives or capers (Or mayonnaise for that matter, but he wouldn't recognize it here) or feta cheese or parmesan cheese, so i'd probably leave the olives out altogether.

WHYM, capers would be a lovely substitute for olives, if you like them I'd try to find something briny like that to use here, as I think it would overall improve the dish. You do have the whole lemon vinaigrette, so you might not miss that briny element too much. I for one would probably toss some good feta, crumbled a bit, into the pasta, passing on the parmesan or pecorino that would otherwise be showered on before serving. ;o)

The olives are just a condiment for the pasta. Leaving them out won't damage the structure of the cooked orzo.

In fact, you could entirely leave out the pasta part and just eat the meatballs by themselves or with a different starch.