Yes, most suppliers can accommodate various dietary restrictions such as vegetarian, vegan, gluten-free, and allergies. Just make sure to discuss this prior to booking.
This can vary depending on the supplier. Chat to them directly if you would like this to be included in your package.
Absolutely, most catering suppliers will do this! But do make sure to ask ahead of booking.
If you have leftovers then you could offer it out to guests to go home with, suppliers at your event or local charities. Chat with your catering supplier as they will have experience in this.