You do realize that the whole reason Sasuke had to wait so long to get his revenge is because everyone kept getting in his way. There was also the whole "revenge is bad" outlook everyone seem to have had during Part One that mysteriously disappeared when it came to Part Two. The fact that...
Between Pain's ability to resurrect defeated Paths and their combined fighting tactics, Sasuke wouldn't be able to defeat him, at least not straight out.
Shikamaru is smarter when he has plenty of intel and the time to plan ahead. But Sasuke's smarter when it comes to figuring things out and coming up with tactics in the middle of the action.
It's probably all about theory over practice. Once you have the theory down, it should presumably work. Also, it's probably based on a lesser technique that they could practice with instead. Basically the same way I imagine Deidara knew his C0, Danzo knew the Reverse Tetragram Sealing, and...
Yes, Sasuke would have been as strong. Kakashi is by all accounts a Kage level ninja, able to go toe to toe with some of the strongest ninjas around and even a Bijuu. So Sasuke sticking around to learn from him would have definitely made him strong. And since Itachi had already come up with...