Charles XII to me is an enigma. I really get the sense that when most historians write about him, they fill in the blanks in his personality, motivations and approach to kingship with how they either feared him to be or how they wished him to be and that really doesn't help fill in a full picture. Then again, when your life reads like a comic book origin story turned Shakespearean tragedy, it's kinda hard to stick to facts.
That being said, I still can't see Charlie taking (and retaining) Poland while being gung-ho Protestant, or at least knee-jerk reflexive Protestant. And yes, I understand that there were differing shades of Catholicism at this time (or any other time) in PLC, but it's just hard for me to see it. There is also the problem of the first parens I put in this paragraph. It's one thing to grab the crown, it's another to retain it. Augustus was the drizzling shits, but as others pointed out, Charlie's rampage did not endear him and his generals really thought massacres were the right way of dealing with locals in Poland (Hell, they boasted about it in their memoirs and letters). PLC notables were not given a great slate of candidates to pick from, but they sorta kinda backed Charlie because he was winning, because everyone loves a winner, except the guys opposing him. So you can sorta kinda talk me into PLC magnates backing Charlie because he's winning and with Peter the Great dead and no yet great, but... Charles in charge of Poland, with his armies and him victorious before all opposition would make a lot of people nervous.
The War for the Spanish Succession is taking place not far away geographically and concurrently and I have to think wigged men in London, Paris, Munich and Vienna would start thinking of whether they want the boy-king on their side and helping them win battles in their war, or whether it would be safer to... well, people have accidents and fall of horses all the time, don't they? Or maybe we can slip a few sacks of gold to Augustus or some plucky Pole with the right heritage to declare themselves King of Poland and back their play in the hopes the boyish-king gets drawn into a long blood soaked civil war.
That being said, if anyone is going to write this timeline, I have only one ask: Charles XII vs. the Duke of Marlborough. Please.