What is sincerity, and what are the benefits of sincerity?

Is it permissible for me to say to non-Muslims "And the same to you" when they wish me a Happy New Year or say Best Wishes?

One of the guys at work was asking me why I became Muslim and I told him it is the path I must follow. I don’t know sometimes what I should say, for me it is the path I must follow. What can I say to someone; I don’t want to push what I believe, so how should I answer someone if asked, what is the right way to answer the questions? Sometimes I feel I don’t give the right answers... Any clues as to what I can say?

I have very difficult question and I think this is my only hope that you can help me to find the answer.
I am a Russian Jewish girl and I know this Muslim guy for over a year. The longer we know each other, the more problems and difficulties we face. Not in the religious or cultural ways. We love each other and there is an issue of whether he can marry me or not.
He is a very good Muslim and he comes from traditional family. I am very fond of his beliefs and beliefs of his family. I was born in a very agnostic country and couldn't be properly introduced into any religion. It was forbidden. When I came to the US I started my research on different religions and found out that my beliefs do not really match all Jewish sayings. I did a lot of research on Islam and by looking at him and a couple other Muslim boys and girls, and by reading Quaran, I have a very strong feeling that I could become a good Muslim. I want to go to school to learn the culture and religion more precisely, to learn the language. I contacted the mosque and was ready to go... but I faced the issue: would I be accepted as a Muslim sister like other people who came from different religions except Jewish? Jews and Muslims are in big contradictions and I know that friendship is almost impossible. I am praying to God (in my home language - Russian), to lead me the right way to achieve my beliefs.
Please, help me to find my answers

How should we call people to Islam?

I am doing research on the concept of monotheism in the world's five major religions. Could you please explain to me the concept of monotheism in Islam?
 

How can a muslim who is really lacking self confidence become a more confident person? He has tried so many things but has yet to overcome his nervousness when talking to people. May Allah reward you with good for your effort.

Why does Islam prohibit adoption? Adoption is a good thing. It provides a home, a set of parents, and a loving family to children who otherwise would grow up without such blessings. Why does Islam prohibit children who have no one from belonging to a family?
 

Is it true that anyone who does not accept that kuffaar are kuffaar is a kaafir himself, even if he prays, believes in the Qur'aan, and the Prophet Muhammad? If so, what is the proof for this? Can a person insist on believing that Jews and Christians can be believers and go to heaven after being shown clear evidence against this, and still be considered a Muslim?

Will a person (a believer) be rewarded if he listens to the Qur’aan without understanding what he is listening to, but he knows that it is Qur’aan? Or will he not be rewarded unless he understands what he is hearing? I hope you could give some evidence from the Qur’aan and saheeh hadeeth.