there may be fewer human-level concepts and more laws of physics

Well, I suppose in principle there might. But would you really want to bet that way?

update - once - in that direction by an appropriate amount

Yes, I completely agree.

capped by the possibility of getting that ratio wrong

Almost, but not exactly. It makes a difference how wrong, and in which direction.

can not [...] give anywhere near the amount of certainty [...] one in a billion

One in a billion is only about 30 bits. I don't think it's at all impossible for the complexity-based calculation, if one could do it, to give a much bigger odds ratio than that. The question then is what to do about the possibility of having got the complexity-based calculation (or actually one's estimate of it) badly wrong. I'm inclined to agree that when one takes that into account it's not reasonable to use an odds ratio as large as 10^9:1.

But it's not as if this complexity argument is the only reason anyone has for not believing in God. (Some people consider it the strongest reason, but "strongest" is not the same as "only".)

Incidentally, I offer the following (not entirely serious) argument for pressing the boom-if-God button rather than the boom-with-small-probability button: the chances of the world being undestroyed afterwards are presumably better if God exists.