Agree 1000000% on the second point. They want to keep people on property, especially for dinner with more restaurants increasing capacity. Figure they can get an extra $15 to $75 of revenue for every guest who stays on property for dinner instead of leaving. [with extremely limited dining capacity, there was less reason to have night time entertainment. But as dining capacity increases, there is more reason to incentivize guests to stay on property through the night]
Whether it increases capacity.... I'm not so sure. The question is whether it gets people to shift their hours versus extend their hours. Then there is also the issue of additional guests attracted to the fireworks. (AP holders, locals, who weren't necessarily planning if there weren't fireworks. Also... guests of Animal Kingdom and DHS who might now park hop to MK at 8pm instead of going back to their hotel.)
I suspect the additional guests PLUS the extenders will significantly out-number the shifters.
The people who prefer morning will continue to be morning people. Though additional may now come back at night.
The commando all-day people will have extra reason to truly stay all day.
Maybe a few of the "late arrivers" will be incentivized to arrive a bit later.
Essentially, I suspect fireworks will increase hourly attendance more than it shifts it. Mornings about the same. Nights much heavier.