If we prove that there is an NP-complete problem that is P, Can we consider that P=NP?
I. I was not able to parse the question, sorry.
II. There is an infinite number of NP-complete problems.
III/I. Each NP-complete problem can be reduced to any other NP-complete problem.
III/II. If you show that there exists a (Karp) reduction from a NP-complete $B$ problem to a problem in P, then you have proved that all problems that reduce to $B$ are also in P. In particular this implies NP $\subseteq$ P, and hence P=NP.
IV. If a NP-complete problem is in P, then P=NP.