Nat offers to take kit away from wethersfield to Saybrook or even the west indies until the trouble blows over why doesn't kit take him up on the offer?

Kit didn't take him up on the offer because she was worried about Mercy. She would never leave unless she knew Mercy was on the mend.

Barbados! The tears sprang to her eyes. "I can't, Nat. I have to stay here."
The concern in his eyes hardened to awareness. "Of course," he said courteously. "I forgot. You're going to be married."
"'Tis Mercy," she stammered. "She's terribly ill. I couldn't go, I just couldn't, not knowing--"
Nat looked intently at her, and took one step nearer. The blue eyes were very close. "Kit--"