So, does that open the possibility for a case to brought up on if the districts were properly drawn? Or is this a completely closed issue?
Nope, that's basically the idea; the case goes back to the lower court for it to re-evaluate them in light of the standard SCOTUS set down.
Is there any obligation for the court to accept (reasonable) facts provided by other parties involved in the lower courts?
It depends on the situation. Usually, appeals courts defer entirely to fact-finding at the trial level unless the lower court's conclusions just seem batshit. But the standard changes sometimes. So with the case we're talking about, there was a motion to dismiss a civil suit. The nitty-gritty can vary, but basically a motion to dismiss is the defendant saying "even if everything you say is true, there's no valid legal claim here." Hence the courts assuming all the plaintiffs' allegations to be true for the purposes of evaluating the motion to dismiss.
This seems to be a pretty obvious outcome, unless I'm missing something?
I don't think you are, and that was kind of my reading to it. The main points of contention among the justices were (a) whether the law regulated speech or not, and (b) whether that's even something SCOTUS should be deciding on its own without having lower courts address that question first.