*"stemmed from a difference in geography." - I'd have gone for "differences in geography" (but not a GA-dependent point)? 
 * "As Karl August Wittfogel points out" - I'd always recommend explaining who someone is when they're introduced, e.g. "As the Sinologist Karl...". Given that he's passed away, I'd also recommend "pointed out" 
 * "Likewise, according to this model, " - I think you're right to emphasise that this is a model (and there are others out there), but if so, it needs to emphasised at the start of the paragraph, otherwise it sounds like undisputed fact. 
 * Not needed at GA, but at ACR or similar I'd be recommending David Sneath's "The Headless State" as an alternative reading of some of the history here. 
 * "Although Manchuria hosted the agricultural lands of the Liao River valley," - I wasn't sure about the "hosted" verb here (the agricultural lands in question couldn't easily move to a different host!) How about "held the agricultural lands" (or contained?)