Instead of us messing about, can you not download a VBR manifest and a corresponding CBR manifest and show the better bitrate in the latter?
I'm not sure I'm following? You asked me how to download a CBR stream from AMZ and I've explained how you do it...?
I'm not here to debate whether VBR or CBR from AMZ is better - regardless of the average bitrate of AMZ VBR streams being higher, 9 times out of 10 the CBR stream is of better quality. And just to confirm, I think this is little to do with CBR vs VBR in general. This is purely down to VBR vs CBR on AMZ.
For 99% of people, the difference between the two will be negligable but it's a fact that the CBR is better 90-95% of the time. There are some older shows/movies/encodes that go against this rule and if you see x264 encoding settings in the mediainfo and it's VBR then it's probably better than VBR (there are still exceptions to this). But it's few and far between....