The Star Wars Sourcebook by West End Games even predates Heir to the Empire. merchandise, the first new companies to put out new products. It came out in 1987. Coming out even before Heir to the Empire, let alone the prequels, there were a lot of unknowns.
Some might assume the later works are contradictory. But that’s not really true. In fact, most of the later works are built on the inventions created by these West End Games Role Playing Games sources. One of the biggest advantages to countering contradictions is that they are written from an in universe perspective. That means one can expect limitations based on what the character writing knew.
Darth Vader, Obi-Wan Kenobi and Yoda all have a brief biography. According to the book, they are excerpt from the Official History of the Rebellion, Volume One by Arhul Hextrophon. The author served as secretary to Mon Mothma. According to the excerpts the information comes from interviews with Obi-Wan, Yoda, Bail Organa and Luke Skywalker. Boba Fett’s information is based on whatever vague sources they can find and they admit being unsure about some of it. Here’s an obvious loophole: if you see a contradiction, the information is clearly presented as coming from people who had a vested interest in keeping the whole thing quiet. None of them are going to come out and admit Darth Vader was Anakin Skywalker. None are going to blare out to the galaxy anything that could reveal a truth that might endanger Luke or Leia.
Mon Mothma, is an example of the prequels drawing from the Expanded Universe. The character who first appears in Return of the Jedi, also has scenes in the novelization of Revenge of the Sith based on scenes cut from the movie. In these scenes the sourcebook revelations are confirmed, that she distrusted Palpatine and was debating with other Senators – including not only Bail Organa, but Padme – what to do about it. Various books and comics reveal that the events involving the changes Palpatine made to the Republic preparatory of making a new Empire and include her in them.
Boba Fett’s account is based on research and in itself admits a lot of emotions. The most out of step element might be the description that his armor is from the Mandaloreans defeated by the Jedi in the Clone wars. This is one of those things Filoni, more than anyone blew up by altering it in the cartoon (for no reason). However the Expanded universe is consistent: the Mandaloreans hired out as mercenaries and cared little about sides at the time. Some were hired by Jango Fett to help train the clones (and being a Mando he is one defeated at the opening of the clone wars.) All the way from Tales of the Jedi to The Old Republic the Mandaloreans have gone to war with the Republic, leading to the Knights of the Old Republic 1 & 2 backstory that Revan was the leader of the Jedi against the Mandaloreans and that it was when he returned from that war he turned to the Dark Side.
The Quarren Seggor Tels apparently brought the shields down on Dac/Mon Calamari to allow the invasion of the Empire. He also talks about how Ackbar and his Council welcomed the visitors from the stars and his regrets. He indicates the Mon Calamari wanted to share things out in the galaxy but his people, the Quarren, did not want that.
Given that all the way back in the Old Republic game, people might assume its a contradiction. But by the era of Knight Errant by John Jackson Miller and the Darth Bane era vast stretches of the galaxy had been cut off from the Republic by war. Dac/Mon Calamari is far enough from the core that it likely would be among those cut off.
What about the fact that they are already in the Republic by the Empire’s arrival if Ackbar was involved in getting them back to the stars? Well given that they never gave a first name that leaves plenty of room. Ackbar is a family name. Admiral Ackbar is Gial Ackbar, his niece was Jesmin Ackbar, who later joined Wedge Antillies Wraith Squadron. Those who welcomed the Republic back to their world might well have been related to him. But it isn’t specified. And even the future Admiral Ackbar, being on a world far from the Core, may well not have known that the new Empire which claimed to stand for security meant to subjugate non human species.
Seggor Tel’s account is a memory. A journal entry, and it’s a single person account. Given that the Republic did come become the Empire that isn’t really much of a leap.
Details such as starships often may seem contradicted. What appears to be an A Wing is in the Droids cartoon, set long before its said to be invented. This was later explained to be the A Wings predecessor, which makes sense in the same way Z-95 and Arc 170s are also predecessors to X-Wings and look similar in design.
Customs and details about species are so common place in the Expanded Universe now we often forget this is where they began: that twilek use their lekku (head tails) for communication, or the wookiee’s honor code.
The Bright Lands and the environment, the cunning of the twileks, the head clans with 5 members are confirmed in: X-Wing: Rogue Squadron books & comics, Tales of the Jedi: Redemption and more.
The wookiee ecology of the tall trees with the it growing more dangerous as you go down.
This is used and confirmed in: KOTOR (game), Young Jedi Knights,Heir to the Empire, the Chewbacca tribute,Legacy of the Force, the Han Solo trilogy by Ann Crispin and Chewbacca and the Slavers of the Shadowlands and more. The wookiee honor code is mentioned in many places regarding Chewbacca especially. It has a lot of fleshing out in the Han Solo trilogy by Ann Crispin.
The Ithorian people and their reverence for mother jungle, their herdships, their devotion to plant life and harmony of the ecology, all appears in later stories.:
Tales of Mos Eisley names Momaw Nadon as a priest that was forced into exile, forced to choose between the destruction of the Ithorian jungle and giving up secrets of their genetic manipulation he was sworn to protect.
In Galaxy of Fear: Spore, we meet his mate and visit Ithor, where it is confirmed that the Ithorians do not visit or allow anyone to visit the surface of the planet.
In Children of the Jedi Leia and Han visit the herdships with their 3 children.
In the New Jedi Order:Dark Tide II Ithor is an important player and has a major role.
The Jawas and their natures:
Jawas and the fact that trading and scavenging is basically their whole culture:
Tales of Mos Eisley
Jedi Academy (game)
The Old Republic (game)
Quarren and Mon Calamari:
The quarren and their disagreement of and with the mon cal also appears in:
Clone Wars (Microseries)
their worship of the trees and other details appear in:
Ewok Adventures AKA Caravan of Courage
Ewoks: the Battle for Endor
Shadows of Endor
Illustrated Star Wars Universe
Gamorrean Culture of males and females:
Murder in Slushtime
Children of the Jedi
Sullustans and their living in caves:
Medstar 1 and 2 (Den Dhur)
Young Jedi Knights