The "marker" I mentioned in a previous post is an arbitrary value you set in an update call, which gets queued up to 20 levels deep, then you can later set a bit for a BLOCKING call waiting for io competion on an update-in-progress using the specified update marker.
There are 16 pixel processors updating those regions in parallel, and although each region can take up to 300 msec to complete, the manual says the eink display can be updated at 48Hz (16 non-overlapping regions @ 300 msec each).
So for the best control, you need to identify your updates with unique markers, then wait on each one before updating again. I suspect you identify regions like specific buttons.
EDIT: Above info is for Freescale eink controller embedded in i.MX508 SoC used in K4 and K5. The K3 uses an external epson broadsheet controller chip with much less capability.