Salaam & Greeting

Replying to salaam correctly

Q: A known person to me sometimes greet me as Assamualekum without la in it, clearly. Sometimes I reply to him in forgetfulness Walekum assalam, instead of Walekum. Then I realize that, and don't know what to read as a counteract to that. Could you please tell me what to do at that times?

Greeting non-Muslims

Q: I work with non-Muslims so the following situations arise:

1. How should I greet them because if I do not greet them then they feel offended?

2. When they say seasons greetings or compliments of the season at December time, how should I reply?