I spent last new years eve at the dh and had a great time and would definatley go back for nye again. The lines for rides were fine in the morning but got much busier later in the day we were staying in the castle club so had vip fastpasses for some rides. We went back into the park about 9.30pm on nye and it was packed(getting close to dangerous) They shut fantasyland and fronterland (dont know about the others) before the fireworks so everbody was on main street, we left and went back up to the castle club lounge to watch the fireworks. The atmosphere in the hotel was great with face painters, jugglers, magicians etc. The parks were quite quiet the next morning as most people opted for a lie in.
I have never been in october but the scottish schools are on holiday from 12 October-23 October and I think the english holidays are from 19 October-29 October so bear that in mind.
Hope this helps