Great questions.
Basically, you are just following the normal baffle step by using the inductor (and in this case a capacitor) to mimic that. It cuts down on parts and complexity. If you were just using a full-range driver, then you could use just a BSC and be fine.
I couldn’t find exactly where I said keep it around 4200hz. But the basic rule of thumb is twice the FS of the tweeter for a second order crossover on the tweeter. If you notice I went from a second order to a third order. Since a third order has a sharper slope, you can crossover later. In this case, I believe this would be fine. Although, the best thing to do is to measure the tweeters performance and distortion.