I wouldn't think that your mk3 would be damaged by what you did. You can't load any roms from the mk3 in the 0.6_A2 beta since the patching routines are broken. Did you try re-formatting the mk3 internal memory with 0.6_A2 before downgrading to 0.5??
You can use menu 0.5, assuming you don't mind that the saving function is basically broken. Otherwise, try menu 0.4 until the final version of menu 0.6 is released (the official .6 should hopefully resolve your problem if nothing else does).
If you are going to downgrade to a lower menu version, please remember to delete the mk2_save file from your sd card, as it's incompatible between the menu versions.
It's also possible to corrupt the contents of the SD card using the beta menus. In that case, reformat the SD card on your pc, re-load the roms, reflash the new menu onto the neo-key, and hook everything up again.
BruceG