hmmm.. i think kabuto dun think he break free tat fast from naruto n he was doing his regenaration (right spelling?) technique so tat he can heal fast... so maybe it explains the 15-20s ... moreover naruto has weird will power to never let go... so even Kabuto was to kick Naruto away.. i dun think Naruto will be flying away like he was.. moreover kabuto was still effected by the Tsunade's nerve attack.. so it will take him longer to figure out which to part of his body to move..

hmmmm i think kakashi will be able to learn Rasengan if someone train him to... Naruto was trained to use Rasengan.. and Rasengan and Chadori are using the same basic... concentrating the chakura on the hand.. anyway i feel.. Rasengan is like the more advance version of Chadori..

And i dun believe Kakashi n Naruto is of the same level... Experience plays a huge part too.. moreover Naruto is too NOISY... i think if Kakashi and Naruto is to really get into a battle.. Naruto will be killed before he can do anything... (cause of his noise level)