There a number of flaws with that plan. First of all, history takes its course whether you want it to or not; there's nothing to say he won't become maddened while dealing with you.

Secondly, there is a chance that Valentine has already informed Sephiroth of his future and thus the man may well be playing you for a fool.

Thirdly, Kadaj has all the intellect of a wet flannel, so it is unlikely that Sephiroth would join his cause anyway. The operational risk is far greater than the possible outcome - rendering this decision on your part utterly unacceptable, as I have already said.