Before I weigh in with my two cents let me declare the caveat that I predominantly race centerboarders in England—feel free to take my views with a pinch of salt.
For me, there are two points. Rule 13 states:
after a boat passes head to wind, she shall keep clear of other boats until she is on a close-hauled course...[my emphasis]
I think the key word there is until. Once a boat passes head to wind, it must keep clear for all the time up to and until she arrives back at close hauled. This is important particularly in starts (in small boats at least) where you might want to lurk on port, spot a gap, tack over, then sit slightly above close hauled on starboard to stop people getting past to windward.
I've learnt the hard way that you don't get your rights back until you hit close hauled...!
The second point is in Rule 18.3:
If two boats were approaching a mark on opposite tacks and one of them changes tack, and as a result is subject to rule 13 in the zone when the other is fetching the mark, rule 18.2 does not thereafter apply...[My emphasis]
Again, I've always understood that to mean that if you tack in the zone (i.e. go beyond head to wind & make yourself subject to Rule 13) the word thereafter
means you can't ever gain rights under Rule 18.2 while rounding that mark.
In the original scenario, I would concur with the opinion that Peter has fouled Paul regardless of how far away Paul is when Peter stalls. It certainly isn't clear or concrete though—I know the RYA publishes a book explaining how the rules should be applied in various situations—and I could well be wrong.
I have to admit though, I would never tend to use 18.3 to get inside a starboard tack boat within the zone. If possible, I would tend to cross in front, overstand to a safe windward position and tack there. This can be a perfectly acceptable position if you think you're going to have to sail high for a bit before sticking the kite up, or if you want to gybe-set on a windward-leeward course and have enough space to blanket the guy in front. At worst, you're in clear air and unlikely to be hauled before a protest committee to explain yourself when you'd rather be at the bar!