Credit card reward points expiry — what is the validity period?

I had the same dilemma. The way I decided was: list down my top 3 monthly spend categories, calculate the rewards on each card, and pick whichever gives the highest cashback on MY pattern. Generic 'best card' lists are useless for personal decisions.

On the add-on card: yes, the primary cardholder is fully liable for all spends on the add-on. There is no separate credit limit or statement. So only issue it to someone you trust absolutely — spouse, parents. Not friends, not siblings you have money disputes with.