To get the six-track mix onto home video without any alteration at all, there would have to be a home video format that supported having five channels across the front. There is no home video format that does this, so it would be both impossible and pointless. The left-center and right-center fronts most likely only contain a mix of the LCR channels anyway, which was only done to fill in the space in a gigantic cinema environment. In a smaller home viewing space, there would be no purpose whatsoever to hearing this, for it would just make everything phasey and weird.
The extra front channels on 70mm not having any discrete information of their own is one of the reasons why, beginning with Star Wars, they were reassigned to the purpose of being dedicated low frequency channels instead.
Absolutely. This took me forever to realize when I first started researching the old formats years ago. Older films generally used them to fill the space of the grand halls playing 70mm as a mixture of L and R stage channels and panned sound/dialogue/across to create a unified stage. This also was due to the fact that they weren’t really mixing a surround styled setup, but something more along the lines of glorified stereo with a dedicated center and a single rear for ambiance to fill the back of the hall. It wasn’t until some of the bigger productions started trying to liven things up that they changed things, and even then it wasn’t until Dolby’s Star Wars remixing of the format to baby booms that anybody did anything about it. I think they were still largely basing this around mixing 4 channel LCRS for premiere Cinemascope engagements and keeping that idea intact for 70. 2001 is pure old school LCRS sounding in it’s original mix. You can plainly hear it on LD because they simply ran the 6 track mix through a Dolby prologic encoder. if you added in two extra speakers in the front and were somehow able to feed them in, it would recreate the proper experience, i.e. they would merely fill in the sound and assist in making the pans work better. Sony’s SDDS did a similar thing in the more modern 5.1 era by having the five channel front return but for mixes that were 5.1 produced.