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.
In general, as bad as the curse is, it also seems to favor keeping the cursed people alive, if only so that they'll continue to suffer — at least, given the sheer magnitude of bad luck they've been hit with (which has included multiple lightning strikes and the first Stanley Yelnats being left to die in the desert), their continued survival would seem to be more than pure chance.
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 (i.e. having Stanley and Zero end up in the same detention facility) to make sure the opportunity would present itself.
The rain is what brought Sam and Katherine together - as the leaky roof gave him an excuse to see her in the schoolhouse and their first kiss happens when it's raining. After Sam dies, it never rains again at Green Lake.
Neither the book nor the film elaborates why Kate spared Stanley Yelnats I. It's established that she killed all the men she robbed and kissed them to boot. Then we remember something: all of the Yelnats are cursed, and Stanley I was Elya's only son. If he were fatally shot, the curse wouldn't pass onto the rest of the Yelnats.
Kate not being explicitly evil is important for the plot. Before Trout shot Sam, she was known to be generous with her time and food. Her spiced peaches were the pride of the town, and she traded them to Sam in exchange for repairs on the schoolhouse. Those same peaches help save Zero's life when they're in the desert because they were on Sam's boat. While she did a lot of bad, the peaches are a reminder of the good she could do during her life, and after her death.
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.
The horrible luck of Zero and his mother suggests they might have a curse going on too. What the hell did they do to suffer that?
It's not necessarily a curse. They've been through some pretty awful years, but it probably all comes down to one incident (whatever happened to make them homeless). What makes Stanley's family different isn't just hard times (a lot of families have bad things happen to them, after all), it's the sheer number of otherwise unrelated misfortunes they've faced.
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.
Also, it's not like she has any kind of preexisting relationship with either of them, she barely knows them except to say hello. From her point of view, she's being asked to choose between two strangers — how would she know that Elya would be a better husband than Igor? It's not like she knows anything about either of them.
In the movie, Pendanski is obviously lying when he says Barf Bag is "still in the hospital". According to "Stanley Yelnats' Guide To Camp Green Lake", Barf Bag ran away from the hospital and disappeared, not realizing his sentence had now finished, but the level of care usually shown to the campers would suggest he's buried in one of the holes. They did say "Plenty of graves to choose from" in the lizard hole scene... It's also almost certainly not the first time this has happened.
Zero's mother not coming back for him implies she was probably imprisoned. She was a homeless woman confirmed by Zero to be surviving by theft, and if she'd been free she would certainly have made every effort to get back to him.
Adding on to this, the book doesn't go into much detail about the situation, but the movie confirms that Zero's mom was looking for him too. Given that theft sentences aren't usually all that long, it's entirely possible that she was released well before Zero went to Camp Green Lake, but it had been long enough that he was no longer staying where she left him, so she didn't know how to find him.