Only reason I said you need one with Wonder Woman is because she introduces a significant mystical element to the universe with gods and such. Also, I think her origin story informs her character in a more profound way than Flash or Aquaman. But yeah, even that you can come back to.
With Flash, I think the strength of his character, at least in the JL cartoons and now in the Arrow-verse, is how instantly likeable he is. You don't even care about not knowing a ton about his origin because you just love the guy. I think introducing Flash upfront, skipping over his rather ho-hum origin so you can jump right into the fun stuff with the Rogues would serve the character better.\
Aquaman being considered a joke is exactly why you don't want to give him his own movie upfront because no one would see it. lol Introducing him in JLA gives you a chance to change perception of the character without risking the IP on a solo film. Now, my "Hi I rule Atlantis" line was me being funny, but I do think there's a way to write it so that it works. Have him claim to be king of the seas but no one believes him. Make it a running joke (people complain there's no humor in DC's films) of people not believing him until he leads an Atlantean army to land and help fight whatever the fuck the JLA is fighting. This is obviously me fanficing, but I think that works.
I think the calling of the character additions to the movie "forced" is premature, as we know nothing about the story at this point. We don't know the capacity in which Cyborg and Wonder Woman are in the story. The nature of Cyborg's "announcement" suggest he's probably not a big role at all.
While I understand WB may not have earned the benefit of the doubt from everyone, I personally just like to have something substantial to chew on before I get upset. I feel like I don't have that yet here. *shrug*