How to convert a ICICI Bank credit card purchase to EMI?

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.

I've owned 4 cards over the last 7 years. The biggest lesson: a card that fits your spend pattern beats the 'most rewarding' card on paper. A 5% cashback card you actually use daily will give you 10x more value than a 10% card you use once a month.