It looks very similar to the current Star Wars a Galactic Spectacular show, projections on the Chinese Theater, Fireworks off center to the right. Here are pictures I have taken from SWaGS and the best spot
They leave a walkway open from Launch Bay to Hyperion Theater so the best spot will be along that walkway so that you have a buffer of empty space. Also you don't want to be too far to the right cause you will miss the fireworks and you don't want to be too forward cause you will miss the projections on the side screens
Best spots are along the walkway, so where the lady on the ECV is or in front of the walkway where the red stroller is.
Early picture (2 hours before the show) from that same spot where the ECV is in the prior picture.I was sitting down
As to how far in advance it will depend on your dates and crowd level. For example, yesterday I was with a group of friends that hadn't seen the SW show but they were also doing CV so my daughter and I saved the best spots for them. We got our spots at 6:15pm for the 8pm show and this front line was almost all taken, it got full by 7pm I am short so I don't mind waiting a long time for a spot if it means I won't have to worry about tall people in front of me
As long as you can see the Chinese Theater and the 2 screens on each side you are good
It is unclear where the dessert party viewing spot will be but if it's the same as the SWaGA one you can see the roped off area on the first pic
There are no reserved viewing area for wheel chair parties in general seating, only if you have the dessert party booked, not sure if that will change with the JBJB show
Hope this helps