It sounds like you're describing this issue. If not, please be more specific here. For example, how are you attempting to 'select a device to control'? Also, make sure to attach your Log.txt file to your reply.
Thanks for clarifying although I'd ask you once again to post your Log.txt file.
I cannot recreate the problem you're describing here in 9.7.4. The left and right arrow buttons navigate between devices just fine. What exactly occurs when you press those on a track that contains devices?
Secondly, the automatic way involves selecting MPD18 as a control surface. Your settings for LPC-Live 2 would remain in place.
"What exactly occurs when you press those on a track that contains devices?" Nothing happens. The buttons for selecting devices (left-right) do not have any effect. Device mode does not show any devices for selection. Only if you manually select the "Lock to Control Surface LPC Live 2" - Device mode will work only with this device regardless of the selected track.