In general MHP is more low key than
MNSSHP. So, there will be lines for the characters, but it's not really like MNSSHP where they recommend getting in line about 45 minutes to an hour before the party starts for the rarer characters!
I concur that Jack Skellington seems to be one of the more popular characters at MHP and the lines can be long. However, he is also out during the day (at least last year he was), so you can try to meet him before the party. He meets over in New Orleans Square...sometimes near the HM, sometimes inside the courtyard in NOS.
3) Apart from the characters, fireworks and parade (and trick or treating) is there anything else specifically for the party that I can't miss, rather than Halloween season?
I would go see the Columbia and Rivers of America area once the party starts. Assuming it the same this year, the dry ice effect is pretty cool. I think there is also a dance party that has lots of characters.
5) Any tips for seeing the fireworks/parades? ie how long beforehand should I find a space and where is best?
For the fireworks, I recommend being on Main Street, beyond the last building, but before the hub. The hub has a good view, but that is usually the most busy and you need to get there the earliest. I usually aim to arrive around 30 minutes before the show starts and have had pretty good luck. I do recommend being able to see the Castle and the area immediately to the left.
For the Calvacade, pretty much anywhere along the route is good. The second Cavalcade is usually less busy than the first. Again 30 minutes is probably a good ballpark figure to find a spot.
6) Any good hidden trick or treat spots? I know, for example, in WDW there's a stretch near tomorrowland that often gets overlooked and has 3 stands right after each other
Almost all of MHP's trick or treat stations are part of a "trail". There are a few stand alone, but not many. The map will show you the locations, so I would use that, and plan on more stations than at MHSSHP. A lot of folks bring a big bag to put the candy in. The bags that they hand out when you arrive for the party and the same sized as the ones at MNSSHP, so they won't hold much candy. People manage to collect pounds of candy...it can get heavy!
7) Is there usually special merchandise and if so where is it? What sort of things do they do?
Last year it was a t-shirt and pins. The pins were actually available during the day (i.e., I didn't need a party ticket to purchase them, which might have been a mistake); the shirts only showed up in the evening. There will also be a lot of Halloween merchandise (not party specific).
As the PP said, once the first party rolls around, there will be lots of information (and pictures) on this thread!
Based on your question about the treat stations at MNSSHP, I'm going to guess that you've been to MNSSHP. There was a thread from a while ago comparing MNSSHP and MHP. It's from 2011, but the 2012 parties were pretty similar, so the comparison is still useful. I say this mostly, just so you can get a sense of the similarities and differences. The parties are very different and if you go to MHP expecting MNSSHP, you might be disappointed. I've been to both parties a couple of times and I enjoy them both a lot. They're different, but both fun. I hope you enjoy MHP too!
http://www.disboards.com/showthread.php?t=2939583