Hi C_ leung
You can complete the return using the UTR number if you are still not in receipt of your NINO.
You are correct in completing the SA100, SA102 and SA109. You would also need to complete the SA106 to claim any Foreign Tax Credit Relief for any tax paid in Hong Kong so that you are not charged twice on the same income.
If the Capital Gain is not taxable then you would not need to report this on the return.
For your wife to share the marriage allowance, please see guidance here:
Marriage Allowance
Thank you.