I am highly confident that the baud rates of my controllers match that of the radio, unless the radio baud rate reported (500k) is not the exact baud rate configured.
You are correct that the standard controllers max out at 115.2k. I have verified this. However, I am using two different USB-UART bridges. One has the Silabs CP2103 chip, which has a maximum supported baud of 1 Mbps. The other is the FTDI FT232 chip, which also has a maximum baud of 1 Mbps.
Having looked at the supported baud rates of both chips, they seem to indicate they support precise baud rates at 500k. And, as I said in my first post, I have verified this by replacing the radio with wires to form a feedback path and verified communication at 500k. And to ensure that they are not in fact configured for, say, 460.8k, I tested one end at 500k and the other at 460.8k and verified communication was not successful. So I have high confidence that the controllers I am using are, in fact, at 500k.
I wouldn't consider this an issue if the radio presented the same problems at 921.6k. But the fact that the radio communication works fine at that baud rate (with the controllers I have) leads me to believe that this is a radio issue itself, and our target baud rate is 500k for this system. If the radio can not support that rate, that is fine, but the documentation says otherwise, so I'm trying to find out the source of this odd behavior at 500k.