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.

Grace alternates between exasperation and dark humor about the denial. By Chapter 219, when even the Lycans have figured it out, the denial becomes absurd comedy rather than genuine tension—though the underlying issue remains.
The "albino lizard husband" reference suggests at least one other supernatural being offers mating. This creates a love triangle element and raises the question of whether Grace's mate bond with Caine is the only option.
Chapter 5, "Caine: She's Human," marks Caine's realization. His wolf recognizes Grace as their fated mate, but Caine is shocked and resistant because she is human—an impossibility in his understanding.
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.
Based on the dynamic, Caine likely finds Grace's defiance both irritating and intriguing. Her sarcasm challenges his authority in ways other wolves wouldn't dare, making her uniquely compelling to a King accustomed to submission.
The quote about stabilizing her lifespan strongly suggests yes. The claiming/mating process may infuse Grace with supernatural vitality, potentially giving her a lifespan closer to that of a Lycan—an enormous incentive beyond romance.
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.
Caine uses his Lycan senses to track Grace's scent trail. In Chapter 41, he pauses at her door to inhale the fading scent, then follows it. His supernatural tracking abilities make it extremely difficult for a human to hide.
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.