Beginning at the end and working backwards, Rule 18 is written to discourage a boat from coming to a windward mark (to be left to port) on the port-tack layline. She is not entitled to mark-room from and must keep clear of all boats on starboard tack. Unless a gap for her appears at the right moment, she's going to be in trouble. That's why we sometimes call it "suicide port". The top sailors all recommend coming into a windward mark on port 5-10 boatlengths away. Pick a spot and tack onto starboard before reaching the zone. Then the rules all work to your advantage at the mark.

If two boats are approaching a windward mark (to be rounded to port) on port tack, then the downwind boat - even if she's clear ahead - is in jeopardy. A boat on her weather hip can control her tack. Even if the astern boat is obligated to keep clear under Rule 12 and owes mark-room under Rule 18.2(b), she can still delay tacking until it's to her advantage.

If you're the clear ahead boat at the zone in this circumstance, then your best option is to scrape the other boat off at the mark. Pinch up or shoot the mark without leaving enough room for the other boat to get between you and the mark. Keep your momentum going as you pass head-to-wind and tack right around the mark. The other boat will have to miss the mark (if so, be careful to avoid her when you tack) or duck you and round behind. If she sticks her nose in anyway, then take avoiding action and protest.

If you have understood the layline and can't prevent the other boat from getting inside, then you're out of luck. If you're overlapped to leeward at the zone, then not only does the other boat control your tack, but you also have to give her mark-room (which then includes room to tack). Your only recourse is to turn head-to-wind and hope the other boat tacks. Then you can tack too, but you're probably behind at that point.

I hope that helps,
Eric