Just visited NYC in April. I had 10 am tour for Statue of Liberty with climb to crown and Ellis Island. We were there until the last return ferry at 4:45. (The crown climb didn't add more than 1/2 hour.) You need to be at Battery Park about 45 minutes before your tour time. It is not a quick trip due to security lines, boarding, deboarding the ferries, second security line, time to get a locker if climbing to the crown and of course time to explore the statue area and much more time at Ellis Island. Ellis island building is more than one level and many rooms have detailed exhibits on how each was used. It takes time to read and see. If you only want to take a quick look at each area you could do a walk by and leave sooner. FYI you need to reserve your tour (ferry) times asap if doing on your own (vs ship tour).
We did the 9-11 memorial on a second day. It is a large museum and you will want to view exhibits, rather than a quick walk through. I think we were there about 2-3 hours. Then did the Freedom Tower Observatory and it was the longest line encountered while on our ten day trip. Even if you get tickets ahead of time, the lines inside the building to get to the tower elevator are significant. The elevators however are amazingly fast. I think maybe it was about 2 hours total with the lines.
As for booking the observatory after Statue/Ellis, I think that would be difficult because you have limited control over the return ferry. When finished exploring, you go to the dock and wait for next ferry. It could have just left the dock and you wait for next one, or maybe full and have to wait for the next one, or you may be fortunate and get on quickly. Allow for travel time and deboarding. Then you have to get to the 9-11 memorial or observatory.
Let me know if you have more questions.