Thanks - I've done some testing on my end and it looks like if my phone is connected via bluetooth to the headunit, whatsapp calls do come through, I can hit Answer, and two way audio works. That notification is coming from the phone, not the headunit's whatsapp.
The Notification Popup app you shared above allows me to see incoming whatsapp and google voice notifications, so it's great to have. The problem is that all notifications I'd previously viewed on my phone from those apps will pop up on the headunit as soon as I start the vehicle, so I then need to skip through them. I don't think there is a way to fix this. There are two other apps I've found that might be better for this purpose: "Bridge - mirror notifications" and Pushbullet. I'm going to test them over the next week.
Hopefully they integrate native app calls/notifications into DUDUOS down the line. It seems like normal Android notification behavior has been disabled in DUDUOS - I'm assuming this is due to some sort of compatibility issue.