I would highly recommend going to MHP, but only if you wanted to do Halloween Party specific activities. The party is a lot of fun (I have a
trip report that has photos and info for the last several years) and there is a lot to do and see in the five hours (plus three of the pre-party overlap). However, if you are only interested in the attractions, then I would probably forgo the party (especially a Tue/Wed party) because it's pretty pricey. MHP has some basic elements that are similar to
MNSSHP/
MVMCP, like an exclusive parade and fireworks show, character meet & greets, other entertainment, and trick-or-treating. If you do go, I would hold off on trick-or-treating until the end of the party...the lines tend to be shorter and then you don't have to carry around all of the candy (each of the treat stops tend to be a "treat trail" with multiple stops so you can can easily get three or four pieces per treat trail. It mounts up quickly.
In terms of mid-September, it's worth noting that DL tends to be operating on a non-peak season schedule during the weekdays (Mon-Thu), meaning reduced hours and less nighttime entertainment. F! and fireworks are usually only on weekends (Sat and Sun, once MHP starts), but WoC is generally nightly. They haven't announced whether PtN or Pixar Play Parade will remain after Labor Day.
The other thing to keep in mind is that there really doesn't seem to be any "unbusy" times anymore. You mention going last in 2005, which is pre-Carsland. It seems like the parks are getting busier and busier, so be prepared for crowds (and lines) on all days. While the weekdays (Tue to Thu) tend to still be quite manageable, it does not mean that the attractions will be walk-on. Even on low crowd days, the most popular attractions (e.g., Radiator Springs, Space, etc.) can still have fairly lengthy lines (>1 hour), so as mentioned, study up on Fastpass and look into Maxpass. FP/MP can help reduce line wait times. Rope Drop tends to be the best option for short lines.
If you really don't like lines or waits, I would consider looking into some of the dining/dessert/reception packages for the nighttime shows...especially for WoC and F!. WoC dessert party is about the only way to get a real seat (the rest of the viewing areas are standing room) and to minimize the wait time. Otherwise, it requires a fairly significant time commitment to get a spot with a good view (you need to be able to see the water surface, which is angled downward from the viewing area). F!, if it goes to the weekend only schedule, is very popular and I would recommend getting either a dining package or, at a minimum, a FP.
