Q: In the preludes Jessica is almost forced on Duke Leto. Yet in the DUNE classic, we read that the Duke actually sent buyers to purchase her.
A: The Leto/Jessica meeting scene in HOUSE HARKONNEN was actually written by Frank Herbert himself and found in his notes. The events shown in HOUSE HARKONNEN are consistent with the original notes.
Sure it was . If it actually was in his notes then he decided it didn't belong in the real story because it conflicted with how he wanted Leto and Jessica's relationship to be understood by the reader.
Q:How are ships powered in the Butlerian Jihad timeframe? Are they using faster-than-light travel? How can a galactic empire hold together if it takes a month for messages to be sent from world to world?
A:The League ships are faster-than-light, but still vastly slower than the near-instantaneous foldspace ships later used by the Guild. Travel between star systems still takes weeks or months. It's the comparative difference between crossing the Atlantic in a sailing ship or on the Concorde. However, historical vast empires -- such as the Roman Empire and the British Empire -- operated successfully even under such constraints.
I think we all know this is one of my personal favorites. They don't even answer the first (and only intelligent) part of the question. Goofs.