In late August, the parks always close earlier than during most other times of the year. We've gone the last week of August for the past 7 years, so it's just something we've gotten used to. That being said, as others have mentioned, it is highly likely the closing times for both DHS and AK will change. When we went last year, though they didn't keep the very late EMH they had introduced when Pandora opened, they did extend the AK closing time to 9:30 pm.
Also, keep in mind, the crowds during that time are generally lower anyway, so I don't think you are going to run into the HEA crowds on a non-party day like you would, say, the week before Christmas. I think, with a good plan, this really won't affect you all that much as I'm assuming you weren't planning on being in MK for all 10 nights of your trip?
If you don't have park hoppers, I would simply avoid the party days at MK. If you have hoppers, I would plan to go to MK on party mornings and then hop to one of the three other parks. Park hours will generally be the same for the other three parks, so, IMHO, the crowds will be equally dispersed. I would also plan on going to MK at least one non-party night to see HEA. It sounds like you have 6 nights to choose from to do this. Disney has events running year-round at this point from festivals in EP, run events, hard ticket events and new attraction openings. I don't think any time frame is immune to having to do some strategic tour planning around these events to make the most out of a trip.
I will also add, we went to the first
MNSSHP last year and it was fantastic. We've now been to 2 MNSSHP and 2
MVMCP and it was by far the best because the crowds were quite low. I'm not surprised they are popular and I'm not surprised they are pushing them earlier so families whose children have to go back to school in late August/early September can go. I was hoping for years to take DS, but was not going to take him out of school to do it. By scheduling them in August, it gave me the opportunity to do so. And it didn't feel "early" to me. WDW offers immersive, environment-changing attractions all over the parks. You board Space Mountain and your are transported to space. You get on Soarin' and suddenly you are flying over Paris. You attend MNSSHP and suddenly it's Halloween. It was completely on par with other WDW experiences.