SOLVED. Thank you all for the help. Over the lunch hour, I experimented some more. Mapping buttons alongside keystrokes was a no-go. However, I figured out if I used just keystrokes, or just buttons---the RPM control would work, even alongside a mapping for an axis. I also discovered that the buttons I was using on the button box, which are rotary type knobs which very briefly trigger a button push, were not holding the press long enough to register movement on the RPM lever. However, using JoyToKey (as suggested by a good fellow in Reddit)--I was able to fix it with this:
Regular buttons would not need this, as one can hold them as long as needed.
Ultimately, I think the breakthrough was using either buttons or key presses---and JoyToKey (which is a great program I've used for a decade)---allows one to leave the key presses at default, have an axis and use buttons.
Thank you again for all the help. What a great community.