I don't think the odds are that high. For GE yes, HS as a whole, not 99%.
I think they'll do something so that only GE reaches Phase 4 so the guests who have zero interest in GE will be able to go to HS.
I'm certain that Disney will use the same messaging that they use at NYE so that most anyone who doesn't know how it works will think it's at phase 4, which will help reduce some of the guests who even try to head over. So that will help cut crowding some too.
We have ADR for a couple of days during opening weekend, mostly to have a place to park ourselves for a bit without having to search for a place while it's so busy. I'm not worried about the ADR since I do think they'll do whatever it takes to get those folks in the park.