I think you're misunderstanding the grandparent comment. Your argument about convection rolls and nonequilibrium statistical mechanics is an excellent argument that the probability of abiogenesis is much, much higher than the probability of some organic molecules in solution spontaneously assembling themselves into a fully-functioning smallest-possible bacteria. However, that's not what the statement that "the odds of abiogenesis are ~1/2^n" is saying.
Saying that "the odds of abiogenesis are ~1/2^n" isn't an assertion of fact, it's merely defining n = -log2(probability of abiogenesis). (Of course, the probability of abiogenesis occurring in any particular body of water is not a constant, it depends on conditions; but all the oceans on Earth are only 80 binary orders of magnitude bigger than a single mL [1], so n for a lab experiment and n for the entire Earth can't be more than a couple decimal orders of magnitude different, so it's immaterial.)
In other words, your argument about convection rolls is an excellent argument that the n for abiogenesis is much, much lower than -log2(probability of organic molecules in solution spontaneously assembling themselves into bacteria), which grandparent claimed is ~1 million. I have no reason to believe ~1 million is accurate, but I don't care, because grandparent has convinced me that the upper bound is many orders of magnitude greater than 73. Your argument that n could be much less than that upper bound is convincing, but unless you're arguing it's so much lower that it isn't much greater than 73, grandparent's counter to the paper still holds.
[1]: http://www.wolframalpha.com/input/?i=log2((volume+of+ocean+i...)