It is likely that swan, as in "well, ah 'swan", is rooted in swoon. Kinda (or is it kindly?) like being overwhelmed.
I've always heard it used "Well, I swanney" meaning "Well I'll be durn"
On another note, an interesting southern phenomenon is how you can say anything negative about anybody as long as you end it with "bless their heart."
That girl is as ugly as a mud fence daubed with tadpoles...bless her heart!"