Things have all been resolved in my favor.
However, it did take finally getting through to someone at the President's Circle desk. Clearly, the first agent didn't understand what my problem was, though I thought I would let his attempt at resolution (dispute the notion that I should have been charged mileage in the first place) work its way through the system. The second agent with whom I spoke seemed to understand my concern a little better ... but he still didn't get it either. What he
did do for me, though, was to connect me to the President's Circle desk. (I had been looking for such a direct line, but I hadn't -- and still haven't -- found it posted anywhere.)
The agent at the President's Circle understood my problem within two minutes of giving him my rental reservation number. And it did not take him much more time than that to confirm that the mileage was what I claimed it had been (roughly -- we were off by three miles

). He claimed that his manager would sign off on the revision the next morning and that I would have a new receipt. This would have been Tuesday, January 14th when we spoke. I think I got the revised receipt a little later than that ... Wednesday afternoon. And the refund for the difference has now turned up on my credit card. So, all's well that ends well ...
... except how do I contact the President's Circle desk in the future? I did take down the number, but I'll probably lose it or it could change. Is it posted somewhere on the Hertz site that I couldn't find? Or do people just call the general line and ask to be transferred? One thing is certain: I won't mess around again with the general line unless I have no other choice.
ETA: I will also note that Hertz was pretty proactive in responding to both my negative survey response and in my interaction with the President's Circle desk. I appreciated the attention they paid to my dissatisfaction. That's exactly the kind of thing that helps me get over such mistakes.