First of all Naruto is faster than Minato and he's 14 or 15 Minato is then faster than raikage who is faster than Lee. However Lee is also only 14 or 15 and was the fastest of his age group until Naruto was able to manipualte the Kyuubi's chakra.
What do you mean all of a sudden? Lee has been the fastest of the Konoha 11 until Recently. He's always trained with weight belts around his ankles. Did you forget the only two able to get close to gaara in pt.1 were Lee and Sasuke. By the time he's an adult, given his current rate of growth, he'll be crazy fast and the fastest character who can't use chakra. Remember Minato was only "fast" because he had a S/T jutsu if it was based on foot speed Minato would be much lower on the fast scale.
It wasn't a decapitation, he cut his body in half. I shouldn't have to explain why that isn't decapitation