I've always been a vocal supporter of multiple characters sharing a legacy title (like multiple Flashes and multiple Spider-Men), but I see where your point of view on this.
It made more sense for Jace to be a Batman that saves Gotham in Future State, as they took place in an undisclosed future where Bruce had to go into hiding. However, you're right, it does feel redundant for Jace to be Gotham's savior when Bruce will still be around. I'm even having a hard time accepting him to be Batman in present-day DC, as he doesn't seem to have a point to be Batman yet. If it was in the future like in FS, then I could believe it, but right now I need more persuasion.
The Green Lanterns really do need a second ongoing, not just a few minis and whatnot. It just doesn't make sense to me. I type that, but I think the issue is that the Green Lantern brand hasn't sold that well in the recent past.
I'm willing to accept Jackson as the new Aquaman because I project his appearances from other media onto his comic book portrayal. Yeah, if I just take his appearances in the comics, it would feel weird for him to suddenly be Aquaman. But if I view him with his history in the Young Justice show and in past DC continuity, then I accept him as being the new Aquaman. I know it still doesn't make it make more sense in a continuity sense, yet I say screw continuity! I rather justify it more meta-textually and emotionally rather than logically. For what they can do with two Aquamans I could see DC doing something similar to what Cates just did with Venom in Marvel. In the most recent comics, Eddie Brock became the new symbiote god and left the venom symbiote with his Son Dylan. He then said they were both Venoms - Eddie will act as the ruler of the symbiote while Dylan becomes the hero on the streets. Something similar can be done with Aquaman, with Arthur being the ruler and face of Atlantis, while Jackson is the street hero.
As for Superman, I don't really have a strong theory on what is going on because I have a strong distaste for grown-up John.