Sorry to hear you are having problems but let me help you with that.
Since you are using a Mirror volume (raid 1) the easiest way to fix it I would say is to access CTRL-I and set the drives to Non Raid Member disks. By doing this you will get a notification saying that you will lose all the information but this does not happen with RAID 1.
So you can set to non raid and then boot the drives and once you are using the Operating System you can re create the RAID with the software.
Try doing this and let me know.
I'm not sure I understand how breaking the mirror changes the "Hard Drive Write Cache Enabled" setting. Are you saying it's not a user adjustable setting? What determines if the setting is or or off? I would imagine if automatically set, the drive's capabilities determine the setting, and the different drive firmware might explain the different setting.
I'm looking to replace the machine in a few months anyway, do you think I should be worried that the settings are different in the same volume?