I'd stay for the parade, during party season the parade actually starts an hour earlier, although I'm not 100% if that change takes effect in August or not. So if it's starting at 2pm, then I definitely think you should watch it.
The next question is a little harder, I think it's enjoyable to see HEA from either within the park or from the beach (or even the pool really). Both viewing areas have their own distinct advantages/disadvantages. You won't be able to see the projections from the Poly, but the fireworks show is just as exciting, and you can relax with a Lapu Lapu or even be in the pool and still see a decent amount. Being in the Hub and watching the projections is much more immersive, but you tend to miss the higher fireworks and it can be incredibly crowded and claustrophobic. At least as a solo traveler, it'll be easier to slip into a spot and move through the crowds. So it really just depends what your goals are.
Worst case scenario, if the parade is a 3pm and you don't get back to Poly until 4pm and you still wanted to go back for HEA at 9pm, it's still a good amount of time to relax, swim, drink. And you can always come back and go to Trader Sam's or Tambu Lounge after the fireworks too since you don't have to be up early the next day! Good luck with your decisions!