Pool luck is not really a measure of luck at all. Pool luck is a statistical calculation, like a probability; so for any given amount of hashing speed at a given difficulty level you should expect a certain return. This is like tossing a coin, if you toss it 50 times you should expect 25 heads and 25 tails, although you may not get exactly these proportions.
Your explanations are good and make sense, however they do not take into account that sh*t can and does happen. I work for a private fund that has lost millions over the last 10 years to outfits like Madoff, Corzine, EDF Mann etc. Yet we are still around, which kinda lets you deduce that there must be substantial funds at work here ;-)
Those were all bona fide, on-the-level, trustworthy outfits...until they collapsed with a ton of money, including ours. And take note of this curious fact: Money never disappears, it's just that somebody else has it. So, without implying that Slush or anybody is skimming, is crooked, or whatever... You cannot blindly go through life like a clown on a minefield, hoping that nothing will ever go wrong and there is no evil.