Another unknown factor to consider is if this ride will have EP by September which impacts standby times too.
Hagrid's Coaster I'm 99.9% sure it not going to have EP by September. It took the original HP rides (due to the insane amount of popularity) years to have them part of the rides to include EP.
By September, at the max, I see the ride being a two hour wait at one point. Could potentially be lower than that, but that is my
max estimate. And even at my max, I could be wrong, as the Harry Potter rides are unpredictable to do this day, sometimes they're relatively reasonable and sometimes, like when I checked a few of weeks ago, it was over an hour for FJ during what I thought would be considered a slow day.
Is there usually EE for the park that has the new ride? I know they schedule EE only a couple of months ahead of time right?
I do not recall them ever restricting certain HP rides from EE since they were new, so I would imagine EE would include Hagrid's Coaster once it's officially open.
As for the topic of SW, I'm hoping people stick to the Disney side for you all (I'm not going near Hagrid's Coaster until Fall)! While I do believe a few people will take advantage of both, I believe if they're truly going for SW they're going to do it during EMH and/or the rope drop for SW, rather than coming to Universal first thing, unless they managed to do SW already and decide to head over to Universal the next day. Personally, my philosophy with newer rides is going at the last minute at night while everyone already left / leaving, or watching the fireworks / the big show in the park.