I like KFC style cheese sauce that is (optionally) served with breaded chicken. And now, at Christmas, I found that a kitchen appliance labeled "raclette and fondue" is at discount. But I made mistakes before and I bought things I do not need because I was not informed. Is a fondue appliance necessary to keep cheese sauce melted? Or will the cheese sauce remain spreadable even if left to become cold? In other terms, can I make cheese sauce with a regular pan and serve it hours later, or do I need something to keep it warm?
To address your title question.:
What is the difference between fondue and cheese sauce
Fondue is typically made with Gruyère cheese, and generally (though not always) is made with wine. Some varieties are also made with some Emmental and/or other cheeses of Swiss origin.
Cheese sauce, on the other hand, is typically made with Cheddar cheese.
do both need to be kept warm to be remain melted?
A fondue warmer is more due to the food being a warm cheesy dipping sauce rather than it simply becoming "too hard" to spread.
Is a fondue appliance necessary to keep cheese sauce melted? Or will the cheese sauce remain spreadable even if left to become cold?
Commercially available Nacho Cheese Sauce is spreadable and does not need a warmer, and depending on the consistency of your sauce when cool/cold this will also be true for yours.
can I make cheese sauce with a regular pan and serve it hours later?
A fondue pot is not necessary to make cheese sauce, so yes you could make it in a pan. You could then allow it to cool, then refrigerate until prior to needing it, then reheat it (so it is not so cold) to the desired temperature and serve without needing a warmer, however its consistency, as already mentioned, will be the deciding factor if you think the warmer will be needed to prevent it from becoming too thick before it is consumed.