Mr. Sir creates a tough, cowboy persona in order to get over the embarrassment of his real name: Marion. This is very similar to Marion Michael Morrison, better known as John Wayne.
Stanley is shown time and time again to have bad luck due to a jilted promise by an ancestor. However, when at Camp Green Lake, he makes not one, but two finds, and manages to survive as long as he did out in the desert, pointing to amazing luck. It makes sense that the curse would let up a bit, because there is now a possibility of a Yelnats carrying a Zeroni up a mountain.
Stanley and Hector have to climb a mountain to get to Sam's onion farm. Did Sam have to do the same? Well, no. When the lake was around, the farm would have been closer to the shoreline. Stanley and Hector had to climb out of the lake's basin.
The story could be a coincidence of epic proportions... or it could be that Madame Zeroni decided that cursing an entire family for all eternity was too harsh and decided to alter the deal in such a way that an opportunity would eventually come up for one of Elya's descendants to break the curse. At the very least, if they were already close, it might have nudged the last few details into place to make sure the opportunity would present itself.
Assuming that Madame Zeroni really had the power to curse people, that means that others can do the same. In that case, who knows how many other families out there are cursed for all eternity? Especially since other curses might not have escape clauses....
Considering the folkloric feel of the book, curses in the setting might work according to Ravenloft rules: they must have escape clauses to take. However, there's no guarantee other cursed families know their curses' escape clauses....
Stanley doesn't know the escape clause either; he carries Zero up the mountain as an act of friendship and survival. The fact that he also breaks the curse at the same time is complete coincidence; Stanley has no idea he's breaking the curse, he just wants to help his friend. So either (1) the curse obligingly made sure (somehow) that everything would fall into place at the right time, or (2) it was pure coincidence. Either way, foreknowledge of the escape clause is clearly not a requirement for stumbling across it.
Myra being unable to choose between Elya and Igor may not necessarily just be due to her being dimwitted, but her being raised in a society where women don't have the luxury of choosing who to marry so when presented with such an opportunity she doesn't even know how to comprehend such a choice.