This Mickey's Halloween Party FAQ post will lead you to lots of info about the MHP, along with links to DIS'ers' MHP reviews and other info -
http://www.disboards.com/threads/ha...ut-halloween-time-2015.3270919/#post-51326916
We don't have any info about the 2015 parties yet, but unless there are drastic changes in store for this year most of the info should still be applicable for now -- or more or less the same.
The MHP
may be going on during the September dates you've narrowed down too, by the way. There has been a rumor that more party dates could be added in this year, and that the party could begin earlier in September than usual. We will probably find out the MHP dates by June, at the latest, or possibly next month
if there are any major changes coming.
As for crowds, a lot of people have stated that they feel the MHP is too crowded considering it is an exclusive event, with such a lofty ticket price. That's not to say that it is as crowded as a typical peak season day in the middle of the summer or anything, but the party is definitely not empty. I think that crowds gather in different locations around the park at different times, so some areas may seem really congested while others may seem reasonable. Some of the lines for rides are short, while others are long.
Also, I think that early weeknight parties (especially Mondays, Tuesdays and Wednesdays) could possibly clear out earlier in the evening due to work and school the next day, whereas the Friday night parties and any party on a weekend would probably be really busy from beginning to end. I expect that a Halloween night party (Halloween is on a Saturday this year) would probably be quite busy.