My DH had that in 2000 and I pulled out some pictures and asked him if he remembered. He thinks (and it looks like it from the pictures) that there was zuchini, summer squash, diced tomatoes, red peppers, cauliflower and brocolli. He says he remembers it being really good!
You might be able to ask for no mushrooms even if they were included. They have been accomidating for us in the past. The only problem might be the vegetables are in a premixed package so they can't sort through them easily. Although depending on the time, the chef might sort through them for you as well.
Probably not the chef. At many restaurants, the servers deal with preparing salads and plating sides. As such, you might want to thank the server for such a service with an extra gratuity.