The future rifle of US is not yet confirmed to be 6.8mm. It is not yet popular round. Many claim that the 6.8mm is useless for everything than useful for anything.
I agree with your view that an intermediate round somewhere between .223 and .308 might be the holy grail post-2020...however with regard to why Army switched back to 308 from the 6.8x43mm, I'm thinking maybe they thought it was too much of a risk? They could not put aside the fact that they now had to rely on OFB & ARDE to not only prove a new rifle, but prove it with essentially a new calibre: one chambered for which there aren't many options to acquire even on the international market.
Even then, with the way Army's requirements are changing, the current 308 too might not be something set in stone yet. I've come to expect that from the Army and MoD.
The future US Army service rifle program (NGSW) seeks a cartridge in precisely this range (more powerful than the 5.56, but lighter than the 7.62). NGSW is meant to be M4 replacement.
Cartridges that buck the trend aren't impossible to adopt on a large scale...look at Chinese PLA's QBZ-95 chambered for 5.8x42mm.
The one in front is a spare magazine holder. Yes it's meant to hold a 2nd loaded magazine, for potentially faster reloading.