Salaam & Greeting

Q: Must one say "As-salāmu ‛alaykum" before entering a room?

Q: Is it obligatory on us to reply if anyone says to us Assalamu Alaikum Wa Rahmatullahi Wa Baraakatuhu? What will be the extent of that reply, do we have to say Wa Alaikum Assalam Wa Rahmatullahi Wa Baraakatuhu or we can just say Wa Alaikum Assalam? Whenever I receives your reply through email, there is always written Salaam in the beginning, do I have to reply to that Salaam as well?

Q: Should we say Aslamu alayk or alaykum to a single person?

Q: What does "was-salaam" mean? Is it Sunnah to end a letter with these words? Is it compulsory for the person who receives a letter which ends with "was-salaam" to audibly reply with wa alaykum salaam?

Q: When I meet my Aunt (father's sister) do I need to bow my head in meeting her or can I just make Salaam from a distance?

Q: If a man greets a woman, is it assalamu alykum or it is assalamu alyka?

Q: I could not hear a person clearly on the phone and replied "Wa alaikumus salaam" to the "Hello"/"Good Afternoon" of a non-Muslim. I thought the person may have said "Assalamu Alaikum", but this is not possible. Am I sinful for this? Should I make Taubah and Istighfaar?

Q: What is the ruling of hugging a friend in Islam? If I am a girl and hug my friend who is a girl too is it permissible? What about hugging your friend everyday at the time of meeting and departure?

Q: Please advise if a niece is allowed to shake hands with her uncles (mums brothers) and clarify all who are allowed or not allowed?

Q: Is it kufr to end a letter with "regards"?