I bought a 6.6 pound can of nacho cheese flavored sauce. It contain a
significant percentage of real cheddar cheese, and has a mild jalapeno
flavor.

What are some good ways to use it in cooking as a base ingredient rather
than as a sauce? Like maybe adding it to some sauteed onion and celery,
and thin with a little beer or chicken stock to make cheese soup?

It's kind of like Iron Chef. ;-)

One thing I'm planning to do is make a stacked (as opposed to rolled)
enchilada hotdish. I just made this up:

Brown a pound of very lean hamburger or ground turkey with a chopped
yellow onion and a twist of black pepper. Add a heaping tablespoon of
hot Chimayo chile pepper and a little chicken or pork stock, and simmer
until almost dry. Do not add any salt because other ingredients later
are salty. Set aside.

Pour a 10 ounce can of red enchilada sauce in a 9x13" lasagna pan. Cut
a package of yellow corn torillas in half so they will stack better.
Thickly spread refried bean on enough tortilla halves to put down a
layer in the pan. Top with a layer of tortilla halves spread with
cheeze sauce. Then spoon on a layer of meat. Repeat. Top with a can
of hot enchilada sauce and grated real cheese. Bake at 325 degrees for
about 30 to 45 minutes.

Best regards,
Bob