Welcome! Congrats on getting the free dining plan--that arrive-on-the-last-day-of-the-promo-but-take-advantage-for-the-entire-trip tip doesn't usually get used very often, so I'm glad you found out about it!!
The EMH debate is a strong one. People have their different opionions, and I think it depends on a lot of factors, such as the time of year you're going, the crowd-tolerance in your particular group, and your previous experiences. The early October time frame usually isn't too bad (that's when we usually go) and we find that EMH's (morning and evening) are fun and not too crowded.
I agree with everyone that you should sign up with Magical Gatherings. Just go on the disneyworld.com website and search for Magical Gatherings, or call Disney and ask about it. It's probably the only way you'll be able to make ADRs for that many folks at one time. Even if you're planning to split up for meals, I still suggest making ADRs for your meals. October is the time of year when the "foodies" go to WDW for the Food and Wine Festival, so the best restaurants are often packed. I already have my ADRs made for the first week of October, in fact! :)
Finally, MNSSHP is a great night and I really think everyone would enjoy it. You can get all the info you need here:
http://www.allearsnet.com/tp/mk/mnssh.htm
There's trick or treating for kids and adults, characters dressed in costume, a spooky (but fun!) Halloween parade, and costume contests for the kids. It's quite an enchanting evening and we've gone for the past three years.
We love answering questions around here, so feel free to ask away!!