There's not a "I want to bring my girlfriend into the country for a few years to see if we're compatible, in which case we may eventually get married" visa. Also, as you know, a fiancée visa would give her three months to marry you or return home. If you're not going to marry within that time, and it will take years before you know if you want to marry, how does that help?

What exactly happened last time? Young people visit the U.S. all the time. Was it pretty clear from the stuff she had with her that she intended to move to the U.S.?