I think it depends on the experience you want to have. Yes, with the projections and lighting around the hub/castle, the hub area is really the best spot to be to take in the 'full' show.
But, HEA is still a GREAT fireworks show and the "entertainment value" of watching from outside the park (Poly, etc) is still quite high in my opinion. Frankly it's a better 'show' watching from Poly than Wishes was. Some of the big fireworks are amazing.
Poly is a great spot because it's close to being a direct view of the castle so has a nice viewing angle between the castle and big fireworks. You have to be a bit careful on the beach because the volume is notoriously low. Stay closer to the path/Hawaii building for better sound (there are speakers in the ground along the path).
As to your specific circumstances, I would be very worried about getting to MK in time with a 7pm Ohana reservation (assuming HEA is 9pm). Ohana runs notoriously behind and it's not unusual to be seated well after your reservation time.
Worst case scenario, you don't get seated until 7:30.... takes an hour to eat/pay (which is pretty fast for some people), you're leaving for MK 8:30...it's going to be a big fight getting up Main Street toward the castle in time. And that assumes no monorail delays, etc. I think that's way too tight.
I'm spitballing a bit here, but unless I were on the monorail leaving Poly at 8, I probably wouldn't want to attempt watching in the MK and would just stay at Poly.
If this were me, I'd enjoy dinner at Ohana, grab a drink from Tambu, and head to the beach after dinner.
If you do have an opportunity to see HEA from inside the MK at some other point in the trip, I would make an effort to do that.
Have fun!