I understand why Strong and other Republican leaders would think the fire-eaters would climb down off the ledge (to mix metaphors) if a Republican were elected President. After all, there was no Republican proposal actually to abolish slavery. All that might have happened is the admission of Kansas as a free state and probable restoration of the Missouri Compromise.
The problem with that assumption is it doesn’t recognize the southern fear of a northern Senate majority, which they saw as fatal to their cause, in the long run. They saw admission of Kansas as a free state as the beginning of the end of their way of life. You’re right that many Republicans probably miscalculated on that score.