I will try to answer your questions to the best of my abilities and knowledge. I may be wrong in some of my answers or misunderstood some information (hopefully another member will correct some of the mistakes that I make or misunderstandings that I have). Here goes:
1) Mata Sati was never ayogya (misfit) for Lord shiv. In fact, everybody wanted their union aside from Daksh, her father. Lord Shiv considered their union to be ayogya because Sati was Daksh's daughter thus was raised in a different environment and believed she would not be adapt herself to his lifestyle and beliefs. Also, Daksh hated Lord Shiv and took every opportunity that he could get to insult him and he had a certain influence over Mata Sati until she herself met Lord Shiv.
2) I believe that both Mata parvati and Mata Sati never knew that they were Adi-shakti before meeting Lord Shiv. Both of them were attracted to Lord Shiv because of the connection that they shared with him. It's just that because of Daksh, Mata Sati never got introduced to Lord Shiv at a young age whereas Mata Parvati had been given a lot of knowledge on Lord Shiv from a young age thus she felt a connection at that age.
3) The purpose of Shiv-Sati marriage was to reunite Adi Shakti with Lord Shiv. Adi Shakti had always been a part of Lord Shiv but he gave her up to Lord Bramha when Lord Bramha was making the universe so Lord Bramha could gain more power or shakti to complete his task. Eventually Lord Bramha felt that Lord Shiv was incomplete without Adi-Shakti so he decided to give her back to Lord Shiv in the form of Sati. Daksh did a great Tapsaya and earned Adi-Shakti as his daughter. Daksh was supposed to reunite Shiv-Sati but his ego eventually came in the way. Sati married Shiv despite her father being somewhat against it.
4) Sati was always "yogya" for Lord Shiv. It's just that circumstances which surrounded these two with Sati going to her father's yagna without being invited and her father's ego/ignorance and hatred towards Lord Shiv eventually forced them apart.
5) He marries her because she is a part of him that he separated from himself and by marrying her, he brings that part back to himself. He is incomplete without her and Sati was always yogya for him otherwise all the other gods would have advised against their union.
CID
