I think that's exactly what the special app is doing, it would be sending the UAC command to change the volume on the dongle. The issue with low volume on Android with the Apple dongle is related, by default it is set to 50% and Android volume buttons only reduce the sample volume. Universal Audio Player Pro is another app people use to be able to control this hardware volume.

My understanding, with BT it is sending a full scale signal either way, some BT devices have the option of their own independent volume but I'm not sure whether you use the volume on the device matters.

With the Qudelix and Android it will show the input at -0dB all the way down to around 35% volume and then below that it does attenuate, but not very much until you get down to the very bottom. I don't think in practice there is any issue using the device volume, not for my devices anyway.