mgpan --
Maybe
figment_jii or
Cheshirecatty can weigh in on this as well.
I don't think going in October is a mistake at all! I think you'll have a great time, even without the MHP! The Halloween Carnival is a lot of fun in and of itself.
October is really only "slower" in that it is not considered the peak season by DLR, and there are fewer people in the parks during that whole time frame than during a peak season. (Random days here and there can be quite busy.) Peak seasons or periods would be Spring Break, summer and the holiday season. September (after Labor Day) and October are technically considered to make up the off-peak season at DLR, but the thing is that Halloween Time has become so popular over the last several years that when you're there, walking around, it doesn't really appear to be slow or off-peak!
Now, that doesn't mean that October is ridiculous New Year's Eve-crazy or something. It's not anywhere near that bad. But there will be a lot of people in the parks at
certain times of that month, and then it will probably quiet down a little bit after mid-month and before the week of Halloween.
There will be a lot of people in
Disneyland during those 3 hours of "crossover time" on party days. So, once the party guests enter DL at 3 or 4 p.m., expect to see a lot of people milling about on Main Street, in New Orleans Square, in Adventureland, etc. If you're concerned that the crowds will be too annoying, you could just leave at 3 or 4 p.m. and head to DCA then.
People have said -- in fact, someone said it earlier in this thread, I think -- that DCA does see an increase in crowds from the exodus of non-MHP people. I think those crowds probably thin out after a while, though...maybe. At the very least, DCA handles crowds a lot better in some ways because many of the paths and walkways are wider and easier to get through, so it doesn't always seem as packed in DCA as it does in DL (in my opinion).