Plant yourself in the middle of the set as soon as you get on. If someone comes by claiming the seat... "oops... I am in A! ohhh, sorry..."

Sit in the middle, so others won't see an advantage of moving into that row.

Noone will take your original correct seat of course.

After everyone boards and if you find yourself with people around you, casually look at your BP, recognize your error, and take your assigned seat.

[heh... OP, you changed your post deleting the specifics... so my response doesnt seem to match... hopefully it helps.]