Yui not being on stage and involved in the group during the entirety of Episode 7 is 100% intentional and according to plan. The blowback by the fans was expected by both Koba and Amuse and they are prepared for it because they are the only ones who know exactly where this is heading.
What we know is that Episode 7 is about the dark side of Babymetal. The light side of Babymetal we have seen as the 3 girls while this "dark" side of the group is manifesting itself differently. The KC show and the video for Distortion hint that there is something seriously off with what is going on and what we are seeing, in actuality, is NOT Babymetal but rather an Evil babymetal fronted by the Alternate Su - the one Su fought onstage in Legend S during Akatsuki.
There are glaring ommisions from the KC show indicating this isn't the Babymetal we know staring with the opening voice over. Gone is the female voice telling us about Babymetal's triumphs around the universe and telling us that it is now time for the metal resistance. Instead we have a darker, evil sounding male voice which changes the tone immediately to a much more grim presentation.
Also there is the opening song. Instead of "Babymetal Death" you get "in the name of". This also indicates that this is not Babymetal as we know it and something much darker. Babymetal Death served as an introduction proclaiming "This is Su-Metal, This is Moa-Metal, This is Yui-Metal, This is Babymetal". The fact that this wasn't performed and at no point did they say during the show "we are.... Babymetal" is because they aren't. Instead the show starts with "In the name of" without any lyrics except for what sounds like the opening male voiceover chanting something indecipherable in the background. Additionally play close attention to the staffs with that circular portal on it and the costumes which also have the circular portal on it. Almost as if they worship this portal. Having the portal also emblazoned on a banner behind the stage adds to this sense of worship. This alternate version of babymetal is fixated on whatever this portal represents. Even the new costumes are darker and more aggressively designed.
The circular portal is seen in the video for Distortion. The Chosen 7, who appear to be the protagonists in the video after literally battling Rock in the video observe the arrival of the portal and end facing towards it as if to battle what will come out of it. Now if they are the protagonists, then this portal and anyone associated with it must be the antagonists. And if you consider the reveal of the girls during "in the name of" with all the imagery as being worship of this portal... Then this version of babymetal are the antagonists, literally their own worst enemy, and fronted by the Alternate Su.
Additionally, who? In the name of who? It's never said. And I'm sure there is a reason why it's not said and will be revealed in the lore via the graphic novel.
Even the Kamis, Gods themselves are corrupted. Gone are their white godly robes replaced with black.
The fact that they announced the Japan shows immediately after the KC show after proclaiming that we don't know when, where, or how the Chosen 7 will appear - as the protagonists to battle whatever this portal represents and this dark version of babymetal - means we actually do know. Doomsday. The final battle. The end of episode 7. The final Japan show. That show will involve this alternate babymetal, the battle between them and the chosen 7 and the return of the light version of babymetal and the return of Yui.