Follow-up question: what proportion of people who fled the civil wars went to the American western frontier vs fully foreign countries?

Still an interesting question, nonetheless. As a follow up question: were the American who fled seen in a negative light, in a similar way a lot of immigrants are seen today in the country of arrival?

I'm afraid that I can't answer your question itself but seeing as /u/Niezo shared it in the Sunday Digest and no-one has been able to, you might find this old answer I wrote about the confederados - southerners who fled to Brazil where slavery still existed - of interest. /u/Bodark43 might find it interesting too, given the link they shared.

I know someone whose Southern family went to Guatemala after the Civil War. As this is not enough to merit a post, I did a search for Confederados and found this in Brazil. I don't think it answers your question, at least not in a scholarly way, but it appears at least some Southerners did leave for places further south.