I raised a similar question 2 weeks ago to the check in agent.
She told me Seat Block is a request not a right or guarantee. If someone with the right priority asks for a certain seat it will be released to them (like MPC members), if someone asks for a seat next to a DM and a SL has a similar seat block the agent will try to honor DMs request first.

One other remarks she made: window seats are open for anyone to request and will be released to everyone regardless of status.