If u want peace of mind and if any shit happen to your phone within 1 year warranty period , better get from fastech, because he can help u to send back china for warranty , other like shopee seller , i not sure how good they handle with the warranty.

China ROM have their own AI , but u will feel a little bit delay because it need send back to china cloud , then resend back to here , sometime fast but sometime might error and have to re-try , while Global unit AI will be base on Gemini / Google AI , so basically it will be more fast.

But Gemini stil can be use je cuma need to install from Play store. I dont use their AI often too , i use Gemini / ChatGPT more in here.
i cant answer about this , u need to reconfirm with Oppo malaysia.

vivo and oppo offer international warranty. but if phone like ultra which not available locally, pazrts will need to order from china. so wait time might be longer. but still better then send back china, risk of lost in mail also.

Normally will have , but might need very fast hand because those super super big voucher will just gone in a second 

Tried so many times without any success ...