I don't think it's about that anymore. A left the door open at the mark and as long as B can get through there and A can't close the door at any point while rounding that mark, B has assumed and taken the inside position legally....that's pretty much the end of the story as it relates to "mark room".

With regards to luffing, it's an overtaking / proper course situation that is completely independent from the mark rounding isn't it?