If I keep Qaza fast for someone else eg. my great grandfather. and club it with mustahab roza of mine as I do not have any qaza roza left.
1. Can the fast be broken like how mustahab can be broken?
2. If broken, will the sawab of the fast kept for individual will be rewarded or it has to be kept like normal fast and cannot break.
3. Will the niyat be wajib qurbat for grandfathers fast and mustahab just qurbatan illallah.

Please guide

1- Yes, you can, even afternoon.
2- If you mean that if someone invited you to a meal, and you broke the qaza fast, are you going to get the thawab, the answer is no
3- It is not wajib.