By Chapter 219, it's become blatantly obvious to nearly everyone—including the Lycans, who "are not the brightest"—that Grace is Caine's mate. Only the stubborn parties (primarily Caine and Grace themselves) continue denying it.

No. Importantly, this is NOT a rejection romance. Caine never formally rejects Grace. Instead, it's a reluctant mates dynamic where both parties resist the bond for different reasons—Caine because she's human, Grace because he killed her pack.
Grace's ex failing to greet the Lycan King at his territory border is a grave insult in wolf society. It may indicate defiance, ignorance, or cowardice—all of which would provoke Caine's wrath.
Though specific physical descriptions vary throughout, readers note that Caine is described as physically imposing—rippling muscles, heaving chest (Ch. 149), powerful presence. Grace internally acknowledges his attractiveness despite her fear.
The chapter describes Grace as "oblivious" with the fan blowing hair across her nose while she sleeps. This suggests she doesn't know about Caine's nocturnal visits, though she may suspect given his behavior changes.
Extremely. Chapter 37 ("Caine: You Touched Her") shows his violent reaction when anyone touches Grace. He also tracks her when she runs (Chapter 41), sneaks into her room (Chapter 221), and maintains constant surveillance.
Caine is caught lounging on Grace's bed, holding her pillow to his face and deeply sniffing it. This reveals his inability to resist her scent despite his cold exterior—the fated mate bond compelling him to seek out her scent even in private.
Yes, multiple times. Chapter 41 shows Caine tracking Grace after she's been gone for two days. Her attempts to flee are driven by her desire for freedom and her horror at being bonded to the man who killed her pack.
Grace forces Caine to confront the limitations of pure power. Her moral objections, her courage despite weakness, and her ability to form bonds of loyalty challenge his "might makes right" philosophy.
Caine doesn't believe Grace belongs in his world. He considers her too fragile as a human and doesn't want to put her in danger. His cold, powerful demeanor masks the internal battle between his sense of duty and the mate bond pulling him toward her.
No. The Alpha is killed during Caine's takeover. His body is placed on top of the pile of corpses in front of the main lodge, as mentioned in Chapter 18. This is a devastating blow to Grace, who considered him her adoptive father.