For Huawei you can change at authorised Huawei mobile center, after this covid MCO is over, just go over and change battery should be rm99 including labour. Last time they do Huawei service day first Friday and Saturday of the month waive labour charge so I only paid like rm60+ for my Huawei p10 battery change.

Yes honor and Huawei same service center. Check the hicare or support app on your phone should have more info on promos

Of course go to official service center would be better. Did mine in sunway pyramid on service day and it was done in less than an hour. Battery last how long depends on how you use it and also luck. 3 years is about at the end of its life if you are a constant user. My wife's still using her Samsung note 2 on her original battery and that is 7 years old already now.

Yeap. You can call them up first to check if they have the battery in stock before you send. Mine they had in stock which is why it was so fast. Also backup your data as they might format your phone and wipe its data. If you say no to that, they will just say whatever happens they will not be responsible. The phone number for their service center should be in your hicare/support app