A boat tacking in the zone is bound by both rule 13 and the additional restrictions of rule 18.3 (if they apply).

No way. It' adding.
When on port, keep clear (RRS 10)
While tacking, keep clear (RSS 13)
After tacking:
1. you are inside (leeward, ROX and inside) but the boat that was fetching the mark should not have to luff over close hauled
2. you are outside (keep clear ou outside), and you must give mark room to the starboard boat.

It's not one or another, it's both, one after the other.

Yes, a boat that is tacking in the zone is bound by R13. The wording in R18.3 refers to a boat that has been on port tack. According to the definitions, a boat that has been on port tack is still on port tack when it goes head-to-wind. If it passes head-to-wind and then has the wind on it's starboard side, then it has passed from port tack to starboard tack but will still have to keep clear of other boats until it has fallen off to a close-hauled course.

Difficult to say that a boat which has not finished its tack is "then fetching the mark," as required for 18.3 to apply.
Only RRS 13 seems to apply. RRS 12 does not apply, so no luffing rights.

Alain,
Which 'right' are you asking about?
R 13 takes out 10, 11& 12.
R 18.3 limits actions.
Boat(s) may be required to 'keep clear' after 10, 11 & 12 kicks in, but sections B, C, & D deliver limits for her.
What could she do, of consequence, as ROW boat ?
Kim

In a dicussion amongst our crew regarding a recent situation at a regatta, i just wanted to clarify the wording in 13 vs 18.3. Taken in isolation, 18.3 seems to give more leeway to a boat passing head to wind from port to stbd. Previous answers in this thread confirms my interpretation that the original obligation when tacking in the zone is governed by 13.

Alain,
No sure how could the two be taken in isolation. The two are tied at the hip.
!8.3 Tacking......
13 While Tacking
Bad things can happen to Port inside the zone with 18.3 ON.
Be careful.
Kim

Kim,
Thanks for the feedback. Yes, port in the zone is tricky. I guess we'll duck next time instead of forcing ourselves in there.

Alain,
I am not saying that you should always 'duck'.
It is all in the art of execution.
I was at the Farr 40 regatta this past weekend and I saw several well executed tacks in the zone just in front of the parade.
I also saw some lost positions for boats coming in on Port.

Just keep in mind; when 18.3 and 13 join forces, the margins for Port are slim.
Fair winds to you!

Kim