I'm buying this, is it the right one?
Yes, that's a popular one to use with my apps.
So I can I send MIDI data from AX8. So I can avoid the AX8 looper and trigger an MP3 instead. Can BandHelper be programmed loop said MP3 and then stop the MP3 when receiving another MIDI signal? What message?
I'm not sure what you're asking here. BandHelper has a basic loop function designed for practice only; it's not sub-second accurate and can't be controlled with MIDI. If you want to perform with audio loops, you'd need a different app for that.
So I enter MIDI hex code into "RAW" field?
You only need to use the Raw MIDI field to send MIDI messages other than song select messages and program change messages. There are separate fields dedicated to those messages.
How do I sync that MIDI data with the backing track MP3?
You would start recording an automation track, then play the backing track and click the buttons to send the MIDI presets at the right times. The automation track captures those actions and their timing.
Are MIDI triggers accurately synced with the audio?
Yes, but if you are playing audio over Bluetooth, that introduces a delay, so you might need to use a wired audio connection. I suppose if you play over Bluetooth while recording the automation track, then any delay would also be captured by the automation timing. But then if you switched to wired later, all the timing would be off. Personally I would use a wired connection unless there's some reason your stage setup doesn't allow that.
I want BandHelper to change the AX8 from Scene 1 to Scene 2 for the chorus. The chorus will be played by the backing track, so this has to be synced properly. Is switching from Scene 1 to Scene 2 a controller change? And what MIDI code does that?
That depends on the device you're sending to, so you should check the documentation for that product to find out what MIDI messages it needs to receive.