Lots of greats on your list! For me, I would put Little Mermaid as a must and Grizzly River Run as a if time permits. For me, Grizzly River Run is very similar to any other amusement park rapids ride, whereas Little Mermaid is a story that I love and the ride is very well done and can only be experienced at Disney. (Same reasoning could go for Monsters Inc as well, but I don't love the Monsters franchise as much as Little Mermaid, but if you love Monsters, then it should make your of musts).
The Animation Building with its attractions are all great if time permits attractions that will give you a great break in your day (as has been said above).
I also wholeheartedly agree that Mater's needs to be on your list as well as lots of strolling time through Carsland both day & night. It's a beautiful land, so well done and has amazing atmosphere and detail that are not to be missed.
The pier at night is also very great but if you are seeing WOC (which is a must for me!) you can enjoy the show & the atmosphere while waiting for the show.
As for Frozen, I have only seen it once on opening weekend and haven't been impressed enough with it to make it back for a subsequent showing, especially for the amount of time it takes to wait & then see the show. Bare minimum you are looking at 90 minutes of time from your day for the show. It would definitely fall into the 'If time permits' category for me, but if you are a huge Frozen fan, it may be more of a must.
As for the Golden Zephyr, it's a very whimsical ride with a beautiful view of the pier but it definitely would fall to the bottom of the list for me because other than that, there's not much to it. Luckily it doesn't ever take too much time to fit in (when it's open), so definitely follow @bswan26's advice and ride it when it's open. It will close not only due to wind, but also for heat (the seats get too hot to sit on) & in the somewhat rare occasion of rain in SoCal, it will close for heavier rain & even rarer lightning. But all those possibilities combined, it's amazing to me that the ride operates much at all!