In my previous post I showed that even without the plausible S5 or Brouwer axioms, we can move from the possible explanation of the maximal simple existential fact (we called it K there), to the existence of a necessary being. Now I see no reason for thinking such an explanation is impossible, and it seems to me that since we’re merely asking for possibility, that premise should get the benefit of the doubt. Regardless, consider this argument for it

1. If each fact in a collection of simple existential facts possibly has an explanation, then possibly the conjunction of those facts has an explanation.
2. Any simple existential fact possibly has an explanation.
3. Therefore, possibly, K has an explanation.

I can’t think of any counter examples to (1) and (2) seems plausible, at least until someone can give us an example of something for which it is impossible to explain it’s existence or non-existence (that is, not merely something that doesn’t happen to have an explanation, something that couldn’t have an explanation). Good luck with that.