Can you get past CMs without a wristband and stay in the park for the parties? Sure. But, there's a reason the parade and fireworks don't start for several hours after the party starts. It gives the CMs plenty of time to get non-paying guests out of the park.
If you don't have a wristband on, you won't be allowed to buy anything, you won't be allowed into a restroom, you won't be allowed on any attractions. If a CM sees you and you are asked to show your wristband and you can't oblige, then you are escorted out of the park.
I actually was behind a family of about 4 or 5 a few years ago...it was at
MNSSHP...heading to the ladies room. I overheard the dad talking to his kids, saying that they needed to stay in the stall for about 15 mins. Then, they could come out and they would be able to stay in the park. Now, it was about 7:30 at this point, the park had been officially closed to non-paying guests for over half an hour. So, this family had managed to avoid CMs for that long. BUT.....as we got to the restroom area, and that family tried to enter the two restrooms, they were asked to show their wristbands. Well...obviously they didn't have any to show. The dad got all miffed and said they hadn't had time to get the wristbands yet. Yeah, right. So, two CMs escorted them to the park entrance, where, I'm assuming, they were made to exit since they hadnt paid for any party tickets.
Yes, it was better when they closed the park at 7 and then herded the party goers into one area, then reopened at 8 for the actual party. But, Disney doesn't feel the need to do that anymore. The CMs are really very good about getting out the freeloaders.
To answer the OPs question...no, not one single thing is free once that party starts. The parade and fireworks are part of the hard ticketed party...if a CM sees you sitting there, waiting for the parade without a wristband, they will take you out of the park.