Okay heres my take on it.

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 think the concept of sin is sort of alien to Sikhi, to my knowldege there are no commandments of things we should and should not do, barring those things that are forbidden to the Khalsa.

I don't think there is anything wrong in a Sikh marrying a non Sikh. My wife and my children are Atheist, it is there choice and I must respect that.

Q2) Is it a sin for a Sikh to marry a non-Sikh and converts to his partner's religion?

Again no, Sikhi says that Sikhi is not the one true way to God, but that all ways are valid, after a fashion. Why would God give us so many ways and then get upset if we decide a differant way was better for us? Keeping God in mind, is the important thing, a non Sikh can do this also.

Q3) Any other comments. Open discussion.

Heh see above.