婚姻登記處問題...?

Yes practically.

Although the Immigration Department will only have access to marriage record in Hong Kong only, given that a Certificate of Absence of Marriage Record (無結婚紀錄證明書) is a known requirement for marriage in the Mainland, the Immigration Department can draw necessary inference that those who have applied previously may have a prior marriage.

當然只有本地人知道，你用自己方法北上，邊個會知道，真係死咗都冇人知！

非在港結婚，婚姻登記應該heck唔到，除非有人報串；但"天網恢恢，疏而不漏"，現實有很多案例因為犯左"虛假聲明"罪行而衰左。