Hi Aldo,
Referring to your interpretations, my opinion is:

Before any pulling/sheet pumping may occur, 42.3(c) tells us that it must be '...possible' to successfully surf, get on to the plane, or foil as a result of those pulls.
In marginal conditions, this creates an implied limit to the number of pulls that may be attempted while the immediate weather conditions remain the same. At some point there comes a time when repeated 'failed' attempts to surf, plane or foil must be taken as an indication that it is not possible.

I'm keen to hear if others agree and their thoughts on what that limit is.

Hi Iuan,
As you point out, it all depends on what's possible.  I will make a decision, with any other judges present, whether surfing conditions exist for the type of boat being sailed. Interestingly, some years ago with Oppies in Weymouth Harbour, the wave pattern and wind were such that surfing was possible on one gybe, but not the other. ;)  Also, with Oppie sailors, skill and body weight will be factors in marginal conditions.  Chord (trough) length is also a factor.  Can you be surfing down the front of a wave if your bow is held up by the back of the wave in front?  Personally, surfing must involve some length of the waterline clearing the surface to qualify as such.
On to planing - most of my R42 experience is with Oppies, 420s, and other youth classes.  So, while I haven't see much of it outside 29ers, initial considerations are, is the wind strong enough, and is this a hull shape that can actually plane?
Foiling - I've only seen from a distance and never had to judge.  I would have to rely on those with the right experience to determine whether these were foiling conditions or not, for the class.  
As for number of attempts in marginal conditions...  The rule I was initially taught was three: first atempt (failed) draws attention; second turns on teh yellow light; third is penalised.  However, I have heard tell of some IJs who will only allow one attempt, though not actually seen it. However, if a sailor is just trying it on, in conditions that have already been deemed not possible, then one attempt might be enough.  If the conditions remain the same and a sailor tries it again, then that probably deserves a second flag and retirement.
But, as always, it depends...

Hi Aldo,

Agreed that the sailor doesn't need to be sailing downwind in order to pump to initiate surfing, planing, or foiling; they can be on any point of sail.

For you other questions, it's worth looking at World Sailing's Rule 42 materials, listing common breaches, interpretations, forms, etc. for a range of classes -- Optis, Lasers, etc.

If we could zero in on the Laser materials:

You're right that 42.c.3.1 states that each sail may be pulled in once. But in Common Breaches for Lasers, PUMP 8 is a penalty for a third unsuccessful attempt to pump. What's undefined: can a sailor can pump twice without initiating surfing, or even three times if the third attempt is successful in initiating surfing, without incurring an on-the-water penalty? Seems like a sailor can, from what Art writes above.

On a related note: it's worth looking at the recommended observation positions listed in Common Breaches for Lasers which World Sailing is "crucial to be able to distinguish between aggressive but legal sailing":

I say this because Lasers may sail very far by the lee to keep their bow from running into the wave ahead, or to sail over a wave. In these cases, they can sail as much 60 degrees by the lee. At this point, their leech will flick multiple times without any pump of the boom. Since the airflow is reversed (going from leech to luff), their leech is essentially starting to luff. If an umpire is behind a sailor, these leech flicks wll look like many pumps and a rule 42 violation. However, if an umpire is properly positioned abeam and to leeward, they'll see that the sailor isn't pumping to initiate these leech flicks and thus in compliance with rule 42. (Luffing a sail is not a rule 42 violation.)

Hi!

If we look at the preamble of WS 42 interpretations-document, I think terms such as "Repeated" and "Yellow light area" becomes critical issues here.

Hi Al,
My reference to three pumps was about three separate, failed attempts on different waves.  If I saw three successive pumps on a single wave, successful or not, that would be a tactical breach and flagged immediately.
Hope that helps...

Ant, thanks, but where do u read about the 3 attempts, to pardon the first, the second and penalize the third only? 

Aldo, here: Common Breaches for Lasers, PUMP 8 

I'm puzzled by Aldo's assumption that surfing only depends on waves and planing, only on gusts of wind.  It is common, when there is sufficient wind, to bear off sharply on the face of a wave, pump the sails hard, and then plane right over the next few waves (or even all the way to the leeward mark).  Similarly, a gust of wind and a pump of the sails can initiate surfing down the wave a boat is on at the time.  In "plain English", the rule says that to initiate surfing or planing in surfing or planing conditions, each sail may be pulled in only once for each wave or gust of wind,

Proviso: That when other boats successfully launch in the conditions, three failures by subject boat merely indicates incompetence (or overweight), not cheating.

Agree about the overweight or incompetence, but also, i do not believe that a boat has a right to one pump,  just because in presence of a wave. 
If no other boat is either planing or surfing, ok, there are no conditions, At third unsuccesful attempt, yellow flag. That's easy.
But also,  if other boats succeeds, i.e. conditions are there, i believe the incompetent boat have not the right to spend is "rightful" amount of pumpings that have by law, one wave/one pump. As Al Sargent make me notice, three unsuccesful attempts are not allowed.
Right?

Hi Rob; sorry for misunderstandng, but i never said, or indended to say, ""that surfing only depends on waves and planing, only on gusts of wind"
Surfing = wave
Planing n foiling: wind 
My point was: pumpings is allowed, not necessarly on a downwind leg only.