1- Allah created everything for the Ahlulbayt (a), who are the most complete human beings. Even the angels were created for them since they possessed the greatest intellect Allah created. As for previous Adams or dinosaurs, we do not know the exact purpose behind their creation, but we do know that they served a purpose since Allah is wise in his creation. There are some possible benefits for us. One benefit is for us to explore previous creatures and study them. Another benefit is to try us. Many people have lost their faith in God because of how evolution is presented. By seeing those “previous Adams” some people thought there is no creator and everything happened randomly through evolution. So it could be a test/trial for us. Also, life is interconnected. The existence of those species may have done something to the environment that would benefit us later. Some people consider the fossils of those dinosaurs as beneficial to us becomes they became oil which we are using today
2- Angels have intelligence, but their knowledge is limited. Allah had not given them all the knowledge. There are many things they know and some things they don’t know. It appears from Hadith al-Kisa’ that in that moment Jibra’eel didn’t know who was under the cloak. Or it’s possible that his question was symbolic. He knew but since it was such an important event he asked Allah who they are (so that Allah would describe their qualities).
3- Animals are made of the earth. They also have an animalistic soul.
4- It seems that most people lived long during his time
5- I have not seen any tradition that states why the size changed.