My advice would be this:
From the All-Stars, take a bus to Downtown Disney.
From Downtown Disney, take the Contemporary/Polynesian bus to the Polynesian for dinner.
One thing to note... I don't know what time your PS for dinner is, and how it corresponds to the fireworks show for that night, but if you're having dinner while the fireworks are going off, that's great. If you're planning on watching the fireworks from the Polynesian beach, I will forewarn you that security will be patrolling the beach. During super busy times, they will be keeping it for the sole use of Polynesian resort guests. If such is the case, my suggestion is to walk over to the TTC, and watch the fireworks from the Ferryboat dock area. The view is excellent, and they pipe in the music. It's a great place.
Now... to get back to All-Star Sports. I would not recommend going back to the Magic Kingdom to catch the bus. Why? It will be JAM PACKED over there. 10x more than usual. My advice would be this... if you're at the Polynesian, head over to the Bell Stand, and have a Bellman call you a taxi. If you're at the TTC, head over to the Taxi stand (just beyond the parking trams, underneath the resort and express monorail beams). A taxi to the All-Stars should be $15 or less, plus tips. (I'm just guessing) It will save you a lot of hassle, and be well worth it.