Based on what you are saying here, I think you have the best plan. If you miss Spaceship Earth, don't sweat it, it is usually a bit slower later in the evening in which you won't really need a FP. If you're going to be at Epcot for the rest of the day on Saturday, you may want to consider switching your FastPass for later. Just because you have already selected that time doesn't mean you can change it. You can go in and modify any of your FPs you choose for other times that may be available pretty much anytime you want. We've even gone in to change times the day of. The Garden Grill is a short walk from Spaceship Earth so you could always make the FP for after your dining reservation.
I'm excited for you to go on the Backstage Animal Tour. We have never done it ourselves but have heard it's great! Since you are really interested in animals, as we are, I know my husband would suggest that you get to AK early and head to Asia to see and hear the Gibbons doing their "calling". He's the only one in my family that has experienced it and says it's mesmerizing…at least for him. They only do it early in the morning which explains why I haven't heard them.
Other must sees in our opinion would be the Festival of the Lion King…getting a FP for this would be key, going to the Flights of Wonder…it will bring tears of joy to your eyes and heading out to Rafiki's Planet Watch which can be very educational. A lot of guests don't care for Rafiki's much since it's more educational, but it sounds like you have such an interest in animals, you could make a day of it. I have to drag my husband out of that place because he'll talk to the CM's about animals, conservation and anything else he can think of for hours.
One other thought, if your nights are free while on International Drive and since you have a car, you could always drive down to Disney Springs to experience that so you can focus on the Parks the 4 days you are there. It's only about 10 minutes away, it's free to visit and free to park. There are a lot of great restaurants, shops and entertainment…especially in the evening. Something to think about.