I agree 100% with ICOT, you can only do what you can do and you are most accommodating to work with your caterer to bring in meals they can eat. As far as chalah, dessert and nosh, that should be left to them. You can choose to tell them that you have arranged a main dish, if they would like to bring anything else make sure it is in its original box with the hechsher on it so the mashgiach can check it before they can bring it to their table.
In that way, they can bring their own cake for dessert and their own candy for their kids. It is not your job to make sure that they are accommodated to the Enth degree. You can only accommodate them to the degree that they can’t do it on their own and that is the main meal. You can also accommodate them with plenty of fruits and vegetables. If they choose not to eat them, that is their choice. So I would say specifically on their table, add extra pickles, olives, tomatoes, kirbies, melons, etc. Nothing that would be questionable like strawberries or lettuce.
You can also ask them to bring extra dessert for the other people at their table so that they won’t feel awkward and the other people won’t either.
It is hard enough putting together a family simcha/party and choosing to pay for it besides. It is unfair of people to make personal requests, no body likes it when they are hosting.