2) Most of the time has been invested by 5k - 2d players. They need much longer for finding moves
Especially with this problem, the usage of TIME makes up for a difference in playing strength.
One example: We needed -- it's long ago, so let's say -- about two months to find a refutation of one of White's strongest alternatives in the top left corner after Black's Guzumi in the top right. Then I had the opportunity to ask Yoon Young-Sun 8p in person about this situation. After a -- may be -- 30 seconds look on the board, she showed me "Black here - White here - Black here", the same sequence in three quick shots that we found after some months' work.
However, there is the danger that stronger players -- especially professionals -- badly underestimate the quality of the results of some amateur Kyu players. Just because they do see only their minor playing strength, but not the large amount of time invested. And do not realize that even amateurs -- on a very small field -- can work with a professional attitude.
In addition, there may be the effect that some of our amateurish Kyu sequences do look very "faulty" or at least "ugly" in higher Dan players' of professionals' eyes, especially when the endgame is concerned. And this may result in an estimation of a low quality overall.
But one has to be aware that -- in my opinion -- this problem is very friendly to amateurs, because it forgives amateurs (but not professionals !) for their mistakes. For example, an endgame sequence of ours may not be correct in absolute terms. We made some one-point-error for Black here, some one-point-error for White there, so our mistakes compensated for each other and our final result was (hopefully) the same as with the correct professional endgame order of moves.
I'd like to suppose that Dosetsu's genius not only was well aware of professionals' strengths and weaknesses, but in addition was also able to put himself in the position of "poor" weak players, and to be "fair" in relationship to their minor skills.
than I would (in rules studies, I see necessary counter-intuitive moves even if they are passes or fill the third eye; I guess I would have found the double guzumi within weeks instead of years because it is an easier move type, serves clear purposes of threatening eyeshape and creating approach defects and fighting in the semeai, I had developed the concept of threatened eye as an alternative to immediate eye in 1998, i.e. before knowing the problem) and they need much longer for finding and checking variations that I would. So the some thousands of their invested manhours translate to maybe 2,500 I would have invested.
Revealing the Guzumi itself was the work of "only" one month's search, not of "years", but finally getting to know about the correct moment, when to play it, took several more months. During this period, Joachim had a lot of work to do to get rid of all the mistakes in my previous ideas and to refute them. But the task itself was relative "simple": "Find a place on the board, where Black can destroy some points of White's territory successfully." This is not at all comparable to the initial professionals' burden of "Try to solve the problem." Probably I was badly handicapped over a long period of time, because I looked for a possibility to further reduce White's territory in Sente only. But the Guzumi is only Gote locally.
Surely, a much stronger player -- given the task mentioned above -- would have found the Guzumi faster. But there was none available
Kang Kyoung-Nang 7d told us recently that the Guzumi would be "naturally" considered by professionals, because it was a "typical" endgame move. Well, why Fujisawa didn't use it in 1982 ? The answer may be quite simple: There was no need to do so. Fujisawa's solution sequence ended in a victory for Black, so it was not necessary to widen the score by using some doubtful bad-shape-move.
Similarly, you will find no hint in the known sources that the main task of the problem cannot be providing Black's big top right group with two eyes. Black would be able to do so within the first 20 moves -- there is no way for White to prevent this -- but thereafter, White won the game by at least five points. Please pay attention that you will not find anything about this fact in the book, we discovered it after printing.
Harry once guessed that -- in principle -- the Guzumi could not be correct, because of the very wild bunch of possible variations in the top left corner thereafter. He mentioned that it is absolutely unusual for a classical problem to establish a lot of pressure to refute this very large amount of possible sequences for confirming the Guzumi as being correct. Possibly Dosetsu considered this a suitable way to "hide" the solution, who knows ?
4) That different people with communication problems have worked at different places means that time must have been wasted for doubled work or void attempts. A single person working alone would have the advantage of efficient work, once the initial 1,000 manhours were invested.
I don’t think that you are correct with these statements.
We three amateurs have our special strengths and weaknesses -- in general or concerning the work on this problem only -- resulting in the effect that our team is much stronger -- in relation to #120 -- than the average of our ranks.
So nearly no time has been wasted for doubled work. But you are right that there have been many void attempts, done by one team-member, refuted by another one. However, the latter is due to our low ranks, and cannot be avoided. And, on the other hand, there have been many ideas, only mentioned by one team-member, worked out in detail by another one. These are the highlights of teamwork.
The "single person" you mentioned above must have a much higher rank than ours to provide the "efficient work" really efficiently. It might be preferable to have at least "two persons", due to the imminent danger of individual blind spots.
But by now, there is no longer the must to invest 1,000 hours in the very beginning. Our book gives a thorough analysis of the problem, and understanding of its pre-conditioned contents -- to get the ability to work on -- must be possible in much less time for a high-class player.
However -- you are right in your review -- the book does not help much in getting stronger, if ever. In principle, it is "only" a thorough analysis of one middle-game position. In addition, no professional can make a living from really solving #120. This is why it is so very difficult to create interest in this topic.
3) From seeing the current book, I get a rough idea of how much remaining work needs to be done. My guess is: another 5d mathematically skilled rules expert's 2,500 manhours but not more.
So altogether #120 I estimate as a problem requiring (me, if I were the person to have attacked the problem after the first 1,000 hours) altogether less than rather than more than 10,000 manhours.
Of course, you would be a great assistance, but don't mess around with this problem. You have other -- by far more important -- tasks to accomplish
In my understanding, the only "big" topic remaining is the validation of the large variation tree of the "Tenuki-plays" after the Guzumi. This is answering the question, whether there is absolutely NO variation for White to refute the Guzumi. A concentration on the "critical" paths (maybe 40) seems to be appropriate and sufficient; there is no need to go over all of the (partially outdated) 1,000 variations. This task is the more time-consuming the lower the rank of the worker is, because the position of the top left corner (and the left side) is so very "open". And there are so many possibilities to go wrong.
As usual -- this is true with #120, too -- it is by far simpler to find an error in an existing sequence (for example Joachim's very decisive (re-) discovery of the "late" Oki in the lower right) than to develop a correct one on your own. This fact was one of our motives to publish the book -- make it possible for others to reveal our mistakes and to improve our solution.