If he had made any moves that appeared to “threaten” the South or the institution of slavery at any point, that would’ve given the South the excuse to move on secession. If Fremont had won instead, that would’ve triggered it 4 years ahead of time. Well, nothing you don’t already know. ;-)
The English Bill could simply be viewed as a partisan action to make KS a Democrat state. It wouldn’t make much political sense for Buchanan to have championed the creation of a Republican state.
The South was going to move to secession anyway. All the rest was pretense.
The property rights issue was becoming too big to ignore anymore. Buchanan today would be named John McCain.