BSOD phone warranty and reviving

I installed a security update a week ago and the phone then worked fine. I went to bed and it was 100% dead when I tried to turn it on a few hours later. I tried every which way to start it, and no pixel/android forum or google chat advice helped. It was 2 days before the end of warranty. It was a grey import phone and google said to pursue the warranty coverage w the Chinese seller.

The phone was purchased on an Aussie platform, Catch.com so Australian consumer law applies conferring 12 months repair/replace/refund rights. The Chinese company that sold me the phone insists on it being sent to them in China and they want the passcode. I've been 100% cooperative w them and supplied all info, documentation & photos they wanted.

Should I send the phone to them? Should I give passcode as they want?!? Are there any other steps I can take to remedy the phone myself???

Thanks!