Kakashi was almost Hokage a few times after the whole Danzo thing. 

And no. Konohamaru's incomplete Rasengan isn't really a good comparison as he, a Genin, couldn't completely use a higher level technique. I mean, using that, it should be proof that Genin typically can't cast high level techniques like that without years of training (as that was 2 years later and he made that small Rasengan). As for defeating a Pain clone, I mean, as a Genin Naruto beat loads of high level Ninja in the series. Just takes a little spam. 

Also, again, Sasuke has the Sharingan. He has the potential to learn just about any technique that he's elementally compatible with. Sasuke is also considered a genius too. Is your character an exceptional genius?