For starters, I also don't think it's just a stop in standard time, I'm not saying it has to do with any kind of subspace, but I think it's something more, because technically it doesn't just affect this world, and when you're in it all the time. what matters is the control of information leaving aside anything material, and about him being inside causality I don't know how to answer, since his being inside is the most obvious but as he stops the time of other universes with other timelines that makes me confused, I can't tell if this is irregular causality or something else.
So far no hints of Acas 5, maybe very maybe Veldanava or Rimuru at the end of LN, and for some reason I started to suspect that Feldway's main body might have it too, as it was said that neither Velgrynd's attack that can kill the real body of one of the angels can affect it but that's just in my head, ignore it.
As for 5D this probably comes from the description of one of the worlds Velgrynd visited during Vol 17, this one might even make sense if the description stays the same when the translation comes out, this Vol is the closest you get to an in-universe Upgrade of slime