1). I believe this is set in the Settings menu. Not adjustable via steering wheel. When using Waze/CarPlay, it is adjustable!
2). No problems here. Maybe try a media system reset? Small hole/button near volume knob. Push & hold w/paper clip for 5 sec or so.
3). Maybe try using CarPlay for music instead? I deleted my Bluetooth settings and do everything I need - music, navigation, phone calls - via CarPlay. Bluetooth connection caused problems.
4). See 2) above. Do you have speed-based volume set in Settings?
5). See 2) above. I don’t use app, ‘cause I don’t have KIA Connect.
6). No idea.