Robin2:
You are not thinking about this logically. What you are doing is the equivalent of watching two german speakers communicating perfectly and then wondering why one of them cannot talk to a french speaker.
But that doesn't make sense to me. Because I have witnessed the "German and French speakers" speak to each other very well in the past with other baud rates. It's not like the HC05 and Arduinos have never successfully interacted: they've worked repeatedly in the past with lower baud rates.
The Arduinos can handle the required baud rate in some form or another because they have done it.
The Arduinos can also handle communicating with the HC-05s because they have done so at lower baud rates.
The only issue is when I have the Arduinos communicating at this level of baud rate through the HC-05s. It doesn't really make sense to suggest that these two devices are completely incompatible (as you compared to the two devices speaking fundamentally different languages) because they have been able to speak to each other in the past.
Robin2:
To my mind that is further evidence for the problem being on the Arduino - over that speed the error becomes unacceptable.
Are you suggesting that the Arduinos are dropping all received information at these baud rates because they determine the error rate is too high for the data to be valid? It's important to note that I'm not getting jumbled data, I'm getting no data whatsoever. Unless the Arduinos are actively dumping this data because it has determined the error rate is too high, I should get something at these baud rates, even if it's gibberish: but I'm not receiving any data at all.
Further, the error should not be unacceptable at those speeds for either device: The Arduinos can handle this kind of speed because they have repeatedly been able to do so via jumper cables/other mediums. The only issue is when the HC05s are included. The HC05s should be able to handle these kinds of speeds because the data sheet for them specifically says that they can handle this baud rate.
I dont' see any evidence that the issue lies with the Arduinos and a distinct lack of evidence, outside of that data sheet, that the HC05s can handle this kind of speed. HC05s have failed to transmit at this speed in every test I have done so far.
Robin2:
Have you carefully studied the Atmega 328 datasheet?
I likely have not studied it as thoroughly as you have. However, I am aware that it does say the max baud rate for an Atmega328p Serial line with an external oscillator providing a 16MHz clock is a 2Mbps baud rate, which is well above my required 480kbps (approx.).
What in the data sheet suggests to you that the Arduino can not handle these kinds of baud rates given that the system has demonstrated to me that it can do so in practical tests?