I think Marx's point in contrasting the "lawful" determination of
commodity values and the "irrational" determination of the money rate of
interest was to emphasize that since interest is a division of surplus
value, it does not represent the realization of labor time in a
commodity. I think he's quite clear on the interest form; the passage
showing how interest comes to appear a cost external to the individual
capitalist is a wonderful example of Marx's way of thinking.
Once you have a money rate of interest it effectively allows firms and
households to transfer value between time periods. The rate of exchange
is formally expressible as the integral of the exponentials of the money
rates of interest between the two periods. This seems to me a formal
point, and not leading to much more insight into the nature of credit or
money markets.
Perhaps the most puzzling thing about the money markets of contemporary
capitalism
is to understand how different national money markets and money rates of
interest can coexist and function effectively as a world money market,
despite there being no world money.