Peter:
The inside leeward boat has touched a mark and therefore broken rule 31. They have an obligation to take a penalty or protest. If they do neither, how can they be exonerated?
Clark Chapin

They can’t be exonerated, but if they don’t call a penalty on themselves and nobody protests, then it’s like a tree falling in the woods. IMO. But we’d expect more integrity from sailors. 

Inside Leeward boat touched the mark and broke RRS 31. No action of Outside Windward boat caused her to break RRS 31. Therefore NO Exoneration for Inside boat. She should take penalty or in this case retire (post race penalty)
Any boat present having seen the incident can protest.
Race committee may protest specially in the light of clear infringement of the rule, as the inside boat apparently did not hail protest or any other indication to outside boat for room to go round the mark. RC if protesting must consider breech of  RRS 2 by the inside boat.

I don't think there are enough facts to decide whether the windward boat broke  RRS 11 or RRS 18.2(b), given the reference to the windward boat's 'belief'. So I will give my opinion with respect to the two alternatives:

If the windward boat did not break a rule - then the leeward boat touched a mark and broke RRS 31 with no grounds for exoneration and her correct action is to retire (or take a post race penalty if one is available) .

If the windward boat did break a rule, then the leeward boat has still broken RRS 31 but she may be exonerated under RRS 43.1(a) or (b), subject to the preconditions of one or other of those rules being met. 

Exoneration is now automatic, so it is not necessary for a boat to lodge a protest to claim exoneration. In the case of the specific scenarios in RRS 43.1(b), this is no change from the 2017-2020 edition of the rule book, all that is happened is the old RRS 21 has been moved to Part 4 and renumbered. 

In the past, exoneration under the more general RRS 43.1(a) (which was RRS 64.1(a) in the 2017-2020 edition of the rulebook) could only be done by the protest committee. As a consequence of World Sailing consolidating the exoneration rules into one place and moving that rule from Part 5 to Part 4, that is no longer the case and it is no longer necessary for a boat to protest to be exonerated. 

Further comments for the sake of completeness.. 

As pointed out above, the discussion is somewhat academic (although conceptually, still very important to properly understand how the rules work) if there is no valid protest. If the incident was observed by the race committee or protest committee (or even the technical committee), they could potentially lodge a protest so long as the validity requirements were met. In practice, this is more likely to happen if they believe there is a possibility that a boat has broken RRS 2. 

A boat knowingly touching a mark and not taking a penalty is the classic violation of the principles of sportsmanship. However, if the leeward boat was deemed by the protest committee to be exonerated under RRS 43 then it obviously follows they have not broken RRS 2 as they were not required to take a penalty or lodge a protest.

If the protest committee found that the leeward boat was not exonerated under RRS 43 then she would be disqualified for breaking RRS 31. It does not necessarily follow however that the boat has also broken RRS 2. If the leeward boat genuinely believed she could have been exonerated by the windward boat's actions, she has not breached RRS 2 even if her belief was mistaken. 

How do you establish whether the leeward boat had a genuine belief they were exonerated?  It's obviously not possible to get inside the skipper's head, so the protest committee would needs to find relevant facts in order to make an inference. For example, establishing what the separation between the boats was.  Other (more circumstantial) factors could possibly include the skipper's racing experience and knowledge of the rules. Notwithstanding the above comments, the fact the leeward boat didn't protest the windward boat could be a relevant factor, albeit not a conclusive fact. It's important to note that under RRS 2 a boat can only be penalised if it is 'clearly established' that the principles of sportsmanship have been violated, so the boat is entitled to the benefit of the doubt to some extent.

TLDR version - as always with any good question, the answer all depends on the facts! 

This is a badly thought out rule, open to abuse, WS should change it to state that Exoneration can only be permitted if the boat that determines exoneration successfully protests another boat that the Jury found to have caused the infringement
Turkeys don't vote for Christmas, WS change this rule.