I find it interesting that one of the most prominent comments, about being a normal and well-adjusted hapa, comes from a guy with an Asian dad and a White mum. This only reinforces the sub's narrative.
Having an Asian father and a White mother is definitely possible. My parents are that as well. But you may have noticed people like us are rare, like unicorns or something. There is a reason for that.
As you say the term hapa is indeed borrowed from Hawaiian, and doesn't necessarily have to refer to someone with mixed Asian ancestry. However, the message of this sub however is most pertinent to a specific mixture and parental combination (hapas with White fathers and Asian mothers). Although this is the primary target demographic, any neutral third party would be able to observe there is something seriously wrong with the relationship between Asians and Whites. Furthermore, the message can be extended to apply to more than just those two groups. I and most likely many others here enjoy hearing from the perspectives of other bi/multiracials who do not have Asian admixture.
I'm sorry that you were "shat upon". Yes this sub can be very abrasive and is not always the friendliest place, but it needs to exist. Many of the people in here have suffered truly terrible experiences, and don't like feeling that their voices are being invalidated. Sometimes that does mean shouting over others. But when it comes to the topic of being hapa, if you have nothing but positive experiences to talk about this is actually a very excellent thing. There are plenty of other places online where mixed people can happily talk about these sorts of things – this is not really one of them. For what it's worth, I have previously looked at other online communities for hapas with a more cheerful vibe. But I've noticed they never last. This sub however is the polar opposite yet it's going strong.
We're not arguing against race-mixing, and no one here is suggesting that all monoracial White or monoracial Asian people should be your enemies. Much of the hate however is directed towards being created from the very worst quirks and aspects of those groups.
I admire your efforts but you can't dispel the negativity in this place, because ultimately this is the reason it was created. The best thing you and I can do is to try and empathise and be supportive, or at the least not throw hate at them. Surely you experienced some degree of confusion regarding your ambiguous race and your identity – I sure did. I think this is a common sentiment among most mixed-race people. But imagine if in addition to this, you received a fuckton of baggage from your parents and society, due to complex factors of racism that were in place before we were even born. This is a place for discussing that.