Shelly-- hum let's see what I remember about the things other than the rides at the party....

Of course the character meet and greets at Halloween are crowded- it's the Villans so you don't see them often- (Maleficent? she freaks me out,LOL) We stayed away from Main Street- b/c honestly- there isn't a lot to do there- shops and bakery. At the
MVMCP they give out cookies and hot chocolate- but how much can you eat of those before you puke?

I'd never watch the parades from Main Street even on a regular night to begin with....

so that doesn't make a difference to me. We found our spot for the parade (the second one- it's less crowded and while everyone is watching the first one you are elsewhere---) about 15 minutes before it started.... yes there were people around us, but no one behind us...
the later you can stay -- the slower it will be. Except on opening night they will be working the kinks out of the season. I know this is gonna sound crazy, but with your trip being the days that they are and the times you might fly back- maybe you should go on Friday night? That way if you had to sleep late on your departure day that would be okay and not interfere with your touring plans too much.

It is gonna be crowded that whole week no matter when you go b/c it's opening week.
Sea is absolutely right- you go for the atmosphere - it has a special feel to it during the party- and if rides are a walk on, it's just a bonus. remember too that once you buy them- you can't get a refund. they mail them to you when you buy them- they are not on your KTTK card.
You could also just plan your days- keeping in mind that you might want to go- and buy them when you get there.
I would not go on opening night b/c many people won't even know about the party until they get there over the weekend and see it's a party going on and buy them then.
okay I'm all over the place with my thoughts I know.

Best day IMO? Thursday, followed by Friday. Unless you don't want it taking up too many days- then I'd do Friday- sleep in Saturday.