Did anyone else notice in the documentary that the Borg cube exploding was remastered in 16:9 and then cropped to 4:3?
Yes if you watch the documentary on Season 1 about remastering, they usually recreate a VFX scene in 16:9 to give them lots of room to work with, then they crop to the best/most esthetically pleasing 4:3 area. TOS-R was done in much the same way.
The original film elements were shot in 35 mm widescreen, then cropped to 4:3 for broadcast.
I noticed the blooper reel was 16:9. and I didn't see any 'junk' off to the sides. Wonder if a widescreen TNG is really that 'impossible'.
Incorrect. They were filmed on 35mm film with standard spherical lenses which has a raw aspect ratio of ~1.37:1 which is just slightly wider than 1.33:1 (4:3). So yes there is a slight bit of more information on the sides, but nothing huge and usually it has equipment and other things never meant to be seen. To film in real widescreen would require framing it that way from the get go, which they did not.
Again this has been explained many times around the web, but also pretty clearly explained on the Season 1 doc.