My report card on many of the topics raised: FAIL
It turns out that if:
+ you really have a circle of competence which is sufficiently large; and
+ for your purposes (don't leverage so can't go bankrupt, income drawn from investment is limited, not otherwise too concerned about mark to market, long time horizon..) diversification and risk management issues associated with it have a very small role to play
...what you are doing becomes ever more optimal. It is so for Buffett and his style is appropriate for BRK which is in that exact position. It is a really big problem when 'you' act like Buffett without having that circle of competence and/or are not in that position.
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Probability is not destiny.
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The largest single determinant for investment success, in addition to latent talent of some kind, seems to be approaching the task in a business-like fashion. Buffett calls it temperament. You'll see that Munger's stuff is full of deliberate thinking rather than seat of the pants got-a-hunch style. You, for example, produced a checklist (somehow confined to things where you've seen fit to award yourself a "FAIL" only).
Apart from being straight-up ripped off, it looks like the biggest killer is over-confidence (an associated one is acting as if overconfident, but really out of desperation to secure a lottery win and escape a bad situation). That seems to reside disproportionately with young males...as if that's a surprise. Another is the opposite of good investment temperament...something like tendency to 'panic'.
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Buffet's way is not the only way. However, it might just be the right way for you. Whatever the case, I'm truly glad that it is working for you (and am when others, who are on a deliberate, heavily rational and considered path to investment/trading success, actually meet with that success). Many paths to Rome. Hope to see you there if I am so fortunate.