Audio normalization across multiple clips and for Youtube, in After Effects I am trying to normalize audio between the clips in my timeline, and at the same time I want to make sure that the overall volume matches Youtube's guidelines.

I wonder if I can read the peak amplitude in AE itself and normalize it across the clips automatically. 

I am not quite sure what I am doing. I've imported a MP3 that should have a reasonable peak volume as it was encoded professionally (not by me), but it sounds a bit too high in volume to what I expected – maybe is there a master volume in AE that I don't know of?

Also, I tried to look up for any guidelines from Youtube in regard to audio normalization, but I had no luck with that, they probably don't exist at all (I wouldn't be surprised, volume on Youtube is all over the place, it's inconsistent even between videos from the same channel).

Thank you in advance.
Your workflow is wrong. You just don't normalize audio with After Effects. Instead either do it with an audio editor, like Audition (which seems suitable to match clips in timeline, just use the normalize effect and maybe a compressor if the dynamic is too great). Finally apply loudness normalization with your video editor. For example during export with Premiere Pro you can set the loudness in the effects tab. Choose a profile and set parameters. For background information on parameter and unit names you can read a PDF here.