Sync multiple devices to iTunes but only one device for playcounts or ratings
I'm pretty sure sure that any time one of your devices is plugged into iTunes it's going to update the play counts regardless of whether or not you have "manually manage music and videos" turned on. More than likely this is because the metadata is completely separate from the fact that you are manually managing the syncing of songs to and from devices. 

I'm not 100% sure if that's the case with the metadata but it seems to be given that play counts are updated on your iTunes library even with "manually manage music and videos" enabled.

The only way I could think of checking that would be to enable "manually manage music and videos" on all of your devices and run a test again to see if play counts update. If they're still updating then I would say that the play counts are part of the metadata in the song file that's transferred to and from your library by the devices even when no actual song files are being transferred or synced between them.