All of this leads me to believe that the "owners" of the lands (and who they may be is an interesting legal question, but the answer doen't change my point, so I won't digress into that) are likely to assert right to developing some of the tract we are talking about. The amount of money at stake would be huge, so a trip to the courts and/or OMB (under existing rules, at that) is likely. Meanwhile, the City may have good reason to argue for the park, but it isn't a slam dunk case in court. And - the solution will be very expensive.
So I'd predict that the City will end up in negotiation with developers (perhaps after a battle over ownership) over how much of this tract is made into a park, versus how much is opened up for high rise development. The City will want to negotiate because a) court is always risky and b) the land left for development mitigates the overall cost to the City.