We have PS for crystal place at 5.40pm on10 Oct. Coincidentally this is a MNSSHP night. My question is, do we get tickets and stay as we are in the park anyway. MNSSHP runs on separate tickets - IMHO it is well worth attending. What we did (and are doing again) is only visiting MK for MNSSHP, we didn't queue for more than 5 mins for a single ride last year, with the exception of Haunted Mansion and that was only because something large had emptied at the same time we arrived.
Is it too late to get tickets or can we get them while we are there You can buy online from Disney.com, they are around $31 for adults slightly less for kids (sorry, can't remember exactly, ordered ours a couple of months ago). The tickets arrived within 10 days via UPS. You have to put your arrival in WDW date on application form to ensure you get your tickets at the right time, but you can purchase over there.
Are they cheaper if you already have a park ticket This I don't know, sorry. Not ever seen anything to suggest they are, but someone may know differently
Will the FP still be in action that night Yes, but we didn't need a single one, everything was walk-on.
Will the park be unbearably busy and will we have an advantage already being in the park There are limited sales of the MNSSHP tickets (I've heard max of 20,000). Whilst it looked horrendous when we arrived at 7pm, by the time we'd arrived at Pirates of Caribbean ride, the crowds were incredibly thin. Really the only time we saw masses of people were at the Headless Horseman's parade and at a couple of the candy stalls to get your free candy. I was dreading leaving the park because of queues, but we walked out, got in our car and were back at AKL within 15 mins - great
We are thinking of arriving at around 5.30-6pm this year - you are allowed to enter a little early and they normally "pen" everyone in a certain area, ie Fantasyland or Tomorrowland" where you can ride all the attractions. This way you're not trying to get in the park with everyone else at the same time. Saying that the lines disappeared very quickly last year. Since you already will be there for your PS you're ready to roll once you've finished eating

I think if you're already in the park earlier in the day and are staying you all get wrist bands to show you're ok for MNSSHP.
It's a magical evening - do you have children?