Rule 10 applies. Rule 18 does not apply as both rule 18.1(a) and rule 18.1(b) apply The two boats are on opposite tacks on a beat to windward (see Case 132). Also the proper course for starboard but not port, is to tack. 
If starboard chooses to hold her course, port must keep clear. There is no rule that requires a boat to sail her proper course.

Surely as soon as it passes head to wind the starboard boat loses its  rights, and if the port track boat then has to take action to avoid a collision or if a collision actually does occur then the tacking boat should lose the protest. When I'm in this situation I shout "hold your course" and spill wind to slow down (as ducking underneath would hit the mark), but sheet in again to miss the stern of the starboard boat. If the boat tacks regardless I protest. 

While S is on starboard she is RoW boat and can stand on.
When S passes head to wind she is now on port. Both boats are on the same tack in the zone so rule 18 applies:
- as the mark has to be left to starboard rule 18.3 does not apply.
- rule 18.2(b) does not apply.
- as S turns from head to wind to close-hauled if she is overlapped inside P then rule 18.2(a) applies. P must give mark-room, and S would be exonerated for any breach of rule 13.
- if S is not, or ceases to be, overlapped inside P then rule 18.2(a) doas not apply and the rules of section A and B will apply.
- while S is entitled to mark room she must still avoid contact (see 43.1(c)

"While S is on starboard she is RoW boat and can stand on"(Gordon Davies)  See case 9

9

Thanks everyone for the responses - you have confirmed what I thought was correct. 

If/when S decides to tack, as soon as she starts to alter she owes P room to keep clear, until S gets to HTW and cedes ROW.

Forcing P to tack and sailing her well past the mark is fairly common in match racing, especially if S is carrying a penalty and would like to create an opportunity to shed it. Less common in fleet racing where S might lose advantage to other boats in the fleet while engaging with P. 

Tim - match racing has a much different rule 18.  S would break 18.3 (see C2.9) if she forced P to tack.

John C .. re: "S would break 18.3 (see C2.9) if she forced P to tack."

Just for my own i-dotting and t-crossing on that statement.. 

In MR:

Angelo - spot on.