nothing but a theory

1) "Theory" doesn't belong in the same sentence
as "nothing but".

Quite true.

Would you call Newton's Laws "nothing but"?

I'd call 'em superceded (by the theory of relativity), but
close enough for government work.

(The modern word for "natural law" is "theory".)

Not true. A "law" is a statement of what happens with
an extremely high confidence -- the laws of
thermodynamics, for example. A theory seeks to
explain, to some degree, how these things happen.
Or, as physics teachers might prefer:

Theories are more certain than you claim - i.,e., believed
to high confidence.

Law = description of a regularly occurring relationship
Theory = proposed interpretation or explanation

In this case, your definition of "law" is less certain than
you claim.

Often when discussing evolution, folks will say something like
"evolution is both fact and theory." They use the word "fact" here
where they probably should use the word "law."

Nope.

What used to be called "natural law" was an attempt at an
explanation of an observation.