I am a complete newbie, and I realised I might have screwed up by joining this forum too late...let's see if you can help me.

My gear:
- Casio Keyboard with Midi and normal audio outs
- Mic: Electro Voice RE20
- Mixer: Yamaha MG10XU (with USB)
- Computer: Macbook Air 2011 with Yosemite

What I would like to achieve:
- Record a podcast in either Garageband (I still keep the last version which includes podcasting) or Adobe Audition
- In my podcast I intend to speak, then decrease volume of my voice to either play with the keyboard or play some music from the computer(normally from Spotify)

What I have done so far:
- Connected the mic to the mixer using XLR to XLR cable
- Connected a pair of headphones to the phone entry of the mixer
- Connected the Mac to the mixer via USB cable
- Connected the Keyboard to the mixer through Line 7/8 inputs (unbalanced)

I did press the USB button on channel 9/10. I did opened Spotify on my mac and can listen to it through headphones, I can also listen to my voice. I did open Garageband and recorded something, and here comes the mess:
- The mic level (despite having level and gain knobs to max) of the recording was very very low and the music was overlapping my voice
- Keyboard sounds ok
- I learnt that the mixer I bought sends out everything from one single USB exit, so it is not possible to adjust voice and music at the same time (in my current setup). I also realised that I am receiving info from the computer to the mixer (Spotify) and then sending the mix out to the computer (I guess this is ok)
- I bough Audio Hijack 3 since it seems you can create workflows and solve that problem, and I was able to boost the level of the mic using the software, but cannot control decrease and increase mic or music volume, etc.

I really appreciate any help to solve the mess, considering what I am trying to achieve. Hope you can help me guys.