There’s a lot more going on behind the scenes than even the author of these epigraphs knows. Reasons why Vin was chosen, and why the power of Preservation needed a new mind to control it.
The author is right in that Preservation did need someone to control its power, and it did seek for a host in which to invest itself. It began this search with what mind it had left about sixteen years before the return of the power to the Well of Ascension, just as it began a search for a new host before the return of the power the previous time.
Unfortunately, just as Ruin took control and manipulated Alendi, he took control and manipulated Vin.
TenSoon and the other kandra resist Ruin and are able to pull the spikes from their shoulders. There are a couple of reasons why they can do this.
The power that Allomancers have to take control of them is the same power Ruin has. That control is exerted in the form of mental pressure through emotional Allomancy. As can be seen from Marsh’s viewpoint, it is more than simply forcing the body to act as Ruin wishes. The extreme pressure on emotions changes the very way the mind thinks, tricking it into doing exactly what Ruin wants. The flaw in Hemalurgists leaves them open to this kind of manipulation.
Kandra, who only have two spikes, are far more difficult to control than koloss or Inquisitors. Vin is able to control TenSoon with ease in book two, but that’s partially because he wanted her to do so. He would have been able to resist her. If she’d continued to push, she could have broken him, but it would have taken time.
Even Ruin’s pressure wasn’t enough to take control immediately. The kandra had a few moments during which they could overcome him and maintain their free will. Beyond that, they were in a cavern surrounded by metal ore in the walls, making it very difficult for Ruin to see what was going on and interfering with his ability to control them.