First of all, the rules at the A (windward) mark are very different than at C (leeward) mark. Rule 18.1(a) says that rule 18 "does not apply between boats on opposite tacks on a beat to windward". If boats on opposite beats are approaching the windward mark, it's simply a starboard/port encounter. If the port boat tacks in the zone (either ahead or inside of the other boat), then rule 18.3 "Tacking When Approaching a Mark" applies. The boat that tacked then has additional obligations, even if she is ahead or leeward.

When approaching a leeward mark, the inside port-tack boat does get mark-room. In the previous (2005-2008) rules, the right-of-way also changed. In the current rules, they do not. The starboard-tack boat retains right-of-way, but has the additional obligation that she must give the inside boat mark-room. When the inside boat gybes, she changes tack, but is still the give-way boat (rule 10 switches to rule 11).

If the two boats are overlapped at the zone, it does not matter which one reaches the zone first. The outside boat must give the inside boat mark-room. See RRS 18.2(b).

If the boats are not overlapped, and the clear-ahead boat reaches the zone first, then the clear astern boat must give the ahead boat mark-room. See RRS 18.2(b).

If the boats are not overlapped, the clear-astern boat reaches the zone first, and they later become overlapped, then the outside boat must give the inside boat mark-room. See RRS 18.2(a).

I hope that helps,
Eric