Thank you for this! It seems like many apps on the growth path go through this same investigation, since we arrived at many of the same conclusions you did (down to calling our own webhooks from within onMessageReceived to retry, or figure out which kinds of devices had the most problems). We spent over a year on this before we hit what I think is our maximum achievable delivery rate of around 85% (short of repeatedly hassling users to go deep into their settings or forcing them to give us an e-mail/phone number to use as backup).

Our app isn't even enabled in China but the points you make in 1 and 2 mean that plenty of devices in South America, Africa, Asia (and even some in North America and Europe) run into these problems.

I think (5) above is a serious unfairness problem since many conversations turn to "I didn't get your notification, let's talk on Whatsapp instead" which affects retention. Now 10-15% failed notification deliveries might not seem like much but if you're talking to 4-5 people either individually or in a group chat then one of them is bound to pull you into a more "reliable" app.

Incidentally we've considered actually reaching out to those top phone providers and ask to be put on the whitelist; not sure if you or anybody on here has tried anything similar.