It's not going to be decided before November. And even if it was, and if it did strike down Obamacare completely, there is no way Congress could get a solution through. The Republicans are too divided and the Democrats have no motivation to solve it before the election.
Also, in the event the courts made a decision, Trump, not the Democrats, would have the greatest prominence and capacity for decision. His natural instinct would be to try to seize the initiative and steal the pre-existing condition issue from the Democrats. I would not bet against his being able to pull it off.
Pull it off how?
Trump could publicly bludgeon Congress to act, blame the Democrats if they fail to do so, and then use his administrative powers to fashion an immediate, stop-gap remedy.