"Q1) Is it a sin for a Sikh to marry a non-sikh even though he stays in Sikhism and his partner remains in her religion? "
i don't think it is wrong but the problem is with the children to whom's religion they will follow.
I know this is a sticking point in many religions. I think some used to insist it was always the woman's religion that determined the child's religion b/c she often had more to do with the "bring up of" the child and the education (religious or otherwise) of the child. Not necessarily true anymore...
"Q2) Is it a sin for a Sikh to marry a non-sikh and converts to his partner's religion?"
i think this is wrong because you have given up sikhi for your partner
Again I think many religions would say exactly the same thing. You have given up Catholicism for your partner to become a Sikh. Or you have given up being a Jehova's Witness to convert to Judaism for your partner. No real difference.
I have always said I feel a true loving god would pardon anyone who has a kind heart and good soul no matter what their belief is in the end. Of course not everyone will agree with me. ;-)
Originally I married a guy who was of Catholic background, but really, non-practicing. His parents wanted me to baptize the kids and I said no. I said when you do that, you make a promise to the church to bring up that child as a Catholic and if we have no intention of doing so I refuse to lie to the church (I have better morals than that in other words ;-)). When they argued I said "Fine...I'll make you a deal...you take the child to church every sunday for 1 year. Then we'll talk about baptizing her." They dropped the subject. Guess it wasn't so important after all... ;-)
so the question for marriage outside religion is always easy for boy and difficuilt for girl.because it is always the girl that give up its religion.
I think this is particular to some religions (you mentioned Hinduism & Sikhism here...) but not all. Just referring back to what I said above about the mom's religion sometimes being the one that is kept due in part to education/upbringing. That certainly was a more European way I suppose...