None of your screenshot show a Windows 10 notification. On my PC I have the "System Sounds" slider set just like in your pictures. Yes, when I click "play" in the "Sounds" tab of the old "Sound" control panel it is affected by the volume mixer. But when
one of the new Windows 10 notifications pops up, it is always at full volume.
So here are the steps to reproduce. In your Sounds tab make sure you re-enable all the default sounds. Then set the "System Sounds" slider to 10% or less. Turn up the volume on your speakers so when you click on the slider you hear the ding very clearly.
Now produce a notification.
Here is one way to produce a notification. Pick one of your USB devices (I would suggest your keyboard). Open the device manager (right click the start button). In the device manager, go the "Universal Serial Bus controllers", and select the "USB Composite
Device" that corresponds to your device. Right click on it, select uninstall, click OK. Then unplug the device. Then replug it. Windows will generate two new-style notifications, one to say the device is being installed, one to say it's done. You'll hear the
volume slider you changed had no impact on the volume of these notifications.
I just did your test with many different notifications that make sounds, even UAC dim desktop with the sound, I have 5.1 stereo speakers on PC that will blow you out of the house. I set my manual volume to 100 , the master volume on PC to 100 and set the
system notifications volume to 5. And just as I said, they played all notification sounds very soft at the 5% volume level. So I do not know what to tell you. You have some sort of issue somewhere or a 3rd party app installed by OEM for sound and volume control.