Is there an resemblance to Christians in the saying: "And upon you be peace and mercy and forgiveness"?

We do not know that Christians greet with the word "Salam" (peace), as it is the greeting of Islam. So, do not imitate them in this. The Sunnah in returning the greeting is to say: "Wa 'alaykum assalam wa rahmatullah" (And upon you be peace and the mercy of Allah), with the addition of "wa rahmatullah." As for adding "wal-maghfirah" (and forgiveness), there is no harm in it. The preferred form of the reply is to begin with the predicate and with the conjunction "wa" (and), "Wa 'alaykum assalam" (And upon you be peace). It is also permissible to say "Salam 'alaykum" (Peace be upon you) without the "wa." The original form of the reply should end with "wa barakatuh" (and His blessings). If the one giving the greeting says "Assalamu 'alaykum wa rahmatullahi wa barakatuh" (Peace be upon you and the mercy of Allah and His blessings), then the addition is obligatory. However, if he restricts it to "Assalamu 'alaykum" (Peace be upon you), then the addition is recommended, based on the words of Allah Almighty: "And when you are greeted with a greeting, greet in return with one better than it or return it in like manner."