[Question] Warranty and purchasing from Japan

Hi, I'm planning to buy few Seikos from Japan through a relative. I have seen that showing passport while buying gives tax reduction and BIC Camera, Yodobashi give it cheaper compared to official Seiko stores/website.

To get the 3yr international warranty, is it mandatory to buy from the official Seiko store?

The billing, if it has his name and not mine, will it cause issue in future warranty claims or servicing (if needed)?

Any other tips or info i should know/convey to the relative for a smooth/better buying experience?

I'm very new to this so any minor advice is very appreciated. Thank you 😄