Since the first boat to reach the zone was clear ahead, Rule 18.2(b) applies. Boat B must give Boat A mark room, and since an overlap was established after Boat A entered the zone, B is also subject to rule 18.2.(c).

18.2(c) When a boat is required to give mark-room by rule 18.2(b),
(1) she shall continue to do so even if later an overlap is
broken or a new overlap begins;
(2) if she becomes overlapped inside the boat entitled to
mark-room, she shall also give that boat room to sail her
proper course while they remain overlapped.

Mark-room - Room for a boat to leave a mark on the required side. Also,
(a) room to sail to the mark when her proper course is to sail close to it, and
(b) room to round the mark as necessary to sail the course.

Proper Course - A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term.

Room - The space a boat needs in the existing conditions, including space to comply with her obligations under the rules of Part 2 and rule 31, while manoeuvring promptly in a seamanlike way.

Therefore, Boat A is entitled to sail their proper course until the overlap is broken.

In practical terms, B can't luff A until the overlap is broken, then re-established. A can sail her proper course, which may be directly to the next mark, or her optimal downwind angle (but she can't jibe, unless that is her proper course to the next mark). It's a very low percentage maneuver for B - until the overlap is broken. If I'm A, I'm going to try to pin B to the mark then say the magic word.