Mixing mushroom-potato-carrot soup and warming later (Should I?)
You're fine making those packets well in advance. You could even put them together the night before and leave them in the refrigerator until you're ready to bake (expect them to take five minutes longer to cook). The only possible negative to doing it that way is that the potatoes could get a little brown. You can keep that from happening too badly by making sure that they are completely covered by a thin coating of the soup mixture. 

You don't want to serve soup made out of the same ingredients as your entree. That recipe is totally scalable, meaning you don't have to make any more than you will actually eat. At worst you're going to have some of the can of soup and part of a potato left over. 

I wish the recipe described the making of the foil packets a little better. If you're at all unsure about how to fold those, come back and ask. We'll help you out.

BTW, that begs for a green salad to go with it. You could put a salad together a few hours before your girlfriend is expected and keep it in the fridge. Just don't add the dressing or any croutons until you're ready to serve.