multiple audio inputs and more than one output layer of audio

I have multiple external audio files (mp3) and a video without audio file (webm) and I wish to use AWS MediaConvert in order to create a mp4 video file.

So far I have succeeded achieving a good result with a single mp3 audio file ("background soundtrack") that plays alongs the entire duration of the video. But here's where things got complicated, I wish to add more audio files on top of that "soundtrack".

For instance I have 2 more mp3 files 4 seconds each with a sound effect.

So, on INPUT I have 3 audio selectors:

• Audio Selector 1 - background soundtrack - external (s3) mp3 audio file (15 seconds)
• Audio Selector 2 - "boom effect" - external (s3) mp3 audio file (5 seconds) [I also set offset of 3000ms)
• Audio Selector 3 - "wow effect" - external (s3) mp3 audio file (4 seconds) [I also set offset of 8000ms)
• Then I created an Audio Selector Group named "Sound Effects" and associate it with Audio Selectors 2 & 3.

Output groups -> File Group h.264

• Output settings: MPEG-4 Container
• Encoding: Video - Default settings with 4M bit rate
• Encoding: Audio 1
• • Advanced Audio Coding, Audio source: "Sound Effects" (Audio selector group)
• • Coding mode 2.0 - L, R (Stereo)
• Encoding: Audio 2
• • Advanced Audio Coding, Audio source: "Background soundtrack" (Audio selector 1)
• • Coding mode 2.0 - L, R (Stereo)

But the result is the video with only Audio 1. From some reason I don't understand it ignores the second audio.

I also wanted the "sound effects" to be "on top" of the "background soundtrack", meaning that the volume of the "background soundtrack" would be "low" so I saw the "Manual audio remixing" feature. But still, it only "attached" the Audio 1.

Questions:

1. In order to have more than a single "layer" of audio, do I have to use the "manual audio remixing" feature or maybe there's another way I have missed?
2. Do I have to set the "manual audio remixing" values for each on of the "Encoding Audio-s"?
3. I didn't quite understand how to use that feature, why there are 2 channels for each audio encoding on default?

Thanks in advance for any help.