"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.

"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

the answer of your above questions also depend on gender.in india whenever a Sikh girl marries hindu boy she also become hindu and children
follow the religion of their father.similarly whenever when a hindu girl marries Sikh boy the girl start following Sikhism and children become. Sikh
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.