Removing the ban for a 90 day period would in no way invalidate the suit, as far as I can see.
Even if an appeal was heard within the 90 day period, it would still be the case that any of the plaintiffs would be denied their rights just as soon as the period ended.
If DC simply followed the appeals court decision and removed the ban completely, then they might be able to prevent an appeal. But if they do that, then they would be admitting to the correctness of the court's decision. They can't afford to do that.
Also, I expect that any replacement system that appears to satisfy the appeals court will, in fact, contain further infringements that can be appealed.
One example would be if any kind of fee is required to exercise the protected right. We don't allow poll taxes and we shouldn't allow arms taxes.
The People's Republik of Kalifornia owes me about a thousand dollars just in fees required of me to pay for proving that I am not a criminal. At such time as the Supreme Court should rule, I will look at suing the state to recover that money.
I did my analysis, assuming DC's goal was, "no guns in DC."
If you change that goal to, "How much can I get for keeping this case out of the USSC?", man. The rules are different.
Fenty can make millions, here. Barry seems to be, already.