I would like to echo Terry Oggel's point and extend it. Whatever the
historical reality of border states in the 1840s, Twain wrote HUCK for the
historical reality of the 1880s.  More than two decades had passed since the
Emancipation Proclamation when Twain released his novel.  In order for it to
be meaningful to his readers - and now to us - it can't be primarily about
slavery.  It can - and is, I think - about race relations in the wake of
race slavery.

That said, Twain always paid close attention to the details of time periods
in which he set his fiction.  His research for historical fiction was
extensive and impressive. While it isn't clear that he researched the
Missouri slave mind of the 1840s as he did Joan of Arc later - he likely
felt he didn't have to, having witnessed that world himself - the historical
sensibility he brought to HUCK seems of a piece with that of his later
historical fiction: accurate, in most places, and plausible within the
confines of his additions to the fictive world.

Excellent discussion, with fine points everywhere.  I look forward to
reading more.

Andy Hoffman