ourherohasfallen , i would not suggest that you flash your player with any other firmwares besides those that you have extracted from the player itself. As you have noticed, when you flashed your other player with a different firmware (ie A to B), the player does not work. Each firmware is player dependent, so very likely it would not work with other players. There is also a chance that if you flash it with a wrong firmware, your player would end up dead, and you'll need to perform a dead player recovery.
Since your firmware versions are v3.5.37, i'll suggest you try to download Waterlily software here
http://www.mympxplayer.org/waterlily...-mb-df256.html
You can reassign the buttons using this software. Then flash the new firmware back into the player.