That'll taste fine, but it's not the same experience, so I recommend the soup.

After a root canal I was told to stick to softer meals. I was recommended tomato soup and grilled cheese sandwich.

I got a fancy feta cheese tomato soup, sourdough bread and 3 kinds of cheese.

Instead of dipping the sandwich I cubed it and dropped it into the soup.

It was heavenly. Reminded me of a Gourmet pizza.

You can but if you don't want soup a marinara sauce is also a good choice for dipping your grilled cheese sandwich into

Yes; and since everything is tastier than tomato soup, tomato sauce will be better too.

The consistency might be a little thick, but you can thin it with some milk to get it smooth.

Yes cheddar and mozzarella is perfect. Processed cheddar may even be preferable especially if it's the kind that comes in individual wrapped slices for sandwiches. Two slices is probably plenty but you can do more if it's larger slices of bread, you just run the risk of the cheese spilling out of the sides while cooking it. If this happens just flip the sandwich onto the cheese until it sticks to the outside of the bread. Garlic salt is also pretty crucial. Low heat and slow is the way to go it's pretty easy to overshoot and burn the bread, if this happens and you catch it in time you can use a butter knife to scrape the black part off until you get to good bread underneath.