My experience trying these on the full (1 hour 40 min) files is that mp3packer64.exe creates a new file with the old filename suffixed with "-vbr". It does seem to be a VBR file (the bitrate on the Foobar2000 status bar jumps up and down by +/- 2 kbps as you play it). It is the same if I try it on a brand new mp3 file I just made on Audacity which is 100% definately CBR to start with.
mp3packer64.exe doesn't change the reported length of the file (as-per the OP) for me - those 3 minute samples I attached still report as 1 hour 57min in Foobar after mp3packering them.
I haven't noticed any degradation in quality (but they are 1.5 hours long and it will take a while to listen all the way through).
I think I'd rather stick with CBRs, though. Originally (15 years ago) I had some mp3 players that struggled with vbr, so I have an irrational aversion to it!
Foobar's 'Fix VBR Header' utility fixes the metadata fine and reports the durations as they should be.
thanks, any further pointers are very welcome