But when not looking at the keyboard, having full height left and right arrow keys makes them feel just like the nearby Option and ? keys. Having a distinct arrow-key-group shape makes them easier to locate by touch.
There's something weird about the way my brain handles the key being the same size as the option key next to it, and the fact that the tops of all those keys are exactly the same.
But focusing the menu bar with Ctrl-F2 does indeed allow me to use Ctrl-f and Ctrl-b. And after hitting Return to open a menu, I can use Ctrl-n and Ctrl-p to navigate down and up.
Interesting.
I will have to try more, but I still think that there are places in macOS where Ctrl-b and Ctrl-f do not work, and I have to use the arrow keys, instead.