You cannot stalk someone in one incident... he followed him. That did not give Martin justification for instigating a physical altercation.
Sure you can. If you are talking about a legal context you might be right (do you have anything other than your assertion for even that) but in the common use of the word, you certainly could stalk someone in a single incident.
It would certainly give someone a reason to instigate a verbal altercation. We don't have any good proof who started the physical altercation and even who started the verbal altercation is in dispute. Zimmerman's claims are the only proof of who began the physical altercation.