Deleted it and all the subsequent Logic10.plists
What other subsequent plists?
There should only be one plus the CS file....
Never delete anything unless you fully understand the consequences...
Did you ensure Logic was not running when you deleted the two files?
If so, what external devices/controllers/keyboards do you have connected to your Mac?
The Musical Typing Keyboard is working fine on all my testbed 10.10.5 and 10.2 systems here so it's likely to be something specific to your setup rather than any kind of bug.. It's just a matter of identifying what that is...
Other thoughts...
Have you tested this with a brand new blank project? (To ensure it isn't a project specific issue)
Have you quit (not just closed or minimized) ALL background and foreground apps when running LPX?