Ok, so that's one way they
could solve this dumb shit. If they keep throwing bodies at Yarlya's jelly cube, then getting killed in her fucked up games, then tortured back to consciousness, they
could figure out a plan and synchronize between the two teams.
Not that I expect them to actually do that though. It's been clearly established that problems can only be solved through sex in this manga. They're probably going to stumble into victory by Yuuta inadvertently cumming on Yarlya's phylactery soul ball or some dumb shit like that.
when did the protagonist completely lose his upper brain and got left with only his lower brain?
I'd say that had already happened in chapter 2.