I have done the Soirée Halloween 3 times, and have visited DLP during the Halloween season every year since 2005 and I personally feel that you may as well go at the very start of the season.
Almost all of the decorations and general Halloween program will be already happening, but there will be a lot less crowds and the weather will be pleasant.
You may miss out on some extra villains, but to be honest, you'll still be able to experience the season but with much less of a crowd

When my husband and I were there last year the queu for Peter Pan's Flight was 15 minutes in the afternoon. I didn't see a single queue in the Disneyland Park over 30 minutes (there may have been longer ones that I just didn't notice of course). No way you'll get queues this short on Halloween.