Well the bottom line is that the emergency exit rows must be occupied by an able bodied person.
That is generally achieved at check in. If nobody is allocated to the emergency exit row (for whatever reason and I wasn't having a dig at check in staff), by the time check in is closed then prior to boarding, the dispatcher is required to reseat somebody in that row.
I will conceed that passengers don't always sit where they should but the onus is on the ground staff to ensure the correct seating prior to boarding.
After boarding the cabin crew must ensure there is somebody in the emergency exit row. However, it is up to the dispatcher to ask the passenger(s) to move from their allocated seat. That's from our ground handling manual.