For missing/wrong items, I had no issues getting support from Swiggy when I complain in a short while after the delivery guy leaves. They either have the missing item delivered free or refund the amount for the item. However its been a while since either of things happened to me. So, not sure how they are handling it these days.
Also, its better to call them up than using the chat.
Btw I still don't understand why Dominos is listed on Swiggy and Zomato. I used to order often from Dominos using Zomato, but one day the order was 1.5 hours late and it wasn't independence day or any such special day, no rains also, but I was unaware that would happen, when I contacted Zomato about Dominos's 30 mins guarantee, they told me "Dominos delivery guarantee is not valid when you order from Zomato". And in the case of Swiggy, I saw that you still need to place a 300 Rs min amount to order from Dominos, and still Swiggy agents don't deliver your pizzas from dominos, it's dominos own delivery guys that deliver your order even when you order through Swiggy.