If I contribute more in both AD-HOC CPF and cash payments to the HDB loan, do I get back the same share I contributed when I sell my flat, or is it based on the 50-50 ownership with my spouse?

Hey! When you sell your flat the amount of CPF you need to return is based on the amount used + accrued interest in each of your respective accounts.

For cash payments, best to work out an internal arrangement between you and your spouse on how to split the proceeds depending on contribution or something :)