Does leaving off the full greeting constitute polytheism (shirk) and apostasy (riddah) necessitating ghusl to enter Islam?

Not returning a greeting of peace (salam) is not an act of disbelief (kufr); rather, it is a failure to observe one of the enjoined religious matters. Returning the salam is obligatory. However, returning a greeting other than salam (such as "marhaban" - welcome) is not obligatory, nor is there sin in not returning it, although it is preferable to respond with similar words. It is not recommended to respond with "wa alaykum as-salam" to someone who said "marhaban." It is appropriate to advise those who do not offer the salam by explaining the guidance of Islam regarding greetings of peace.