Is the zabiha (prescribed method of ritual slaughter) valid if the person doing it does not offer salah regurarly? I ask because I heard a person who misses prayers without a valid reason would be considered a kafiir.
No. If the person believes in Allah and His Oneness then he is not kafir. Not offering prayer occasionally or chronically doesn't deem him as kafir/infidel.
Prayer is an obligatory thing, ordained by Allah, for all Muslims. Not offering it will make him accountable as a Muslim.
p) implies salah (