Now a new book about Eric Chahi has been released, I finally have the answers to all of my questions. The book spends 2 pages to explain all the production of the score for Heart of Darkness, with a full picture of Bruce Broughton conducting the score.
Eric Chahi started the (very long) production of Heart of Darkness in 1992, following his Another World/Out of this World hit game. He wanted to have the first orchestral score in the history of video games and so he logically wanted to hire a film composer to score his game. He had been really impressed by the music of Young Sherlock Holmes and more recently by Rescuers Down Under (1990) and contacted the agent of Bruce Broughton in 1994 (once he had rough animated sequences ready to be scored). Bruce agreed to come in Paris to see the animated sequences and discuss with Eric. He was impressed by the game and got very interested in this atypical project and much inspired to write all the wonderful themes and cues needed. The recording sessions took place in London in january 1995 with the Sinfonia of London. Eric Chahi recalls that seeing the orchestra giving life to the score of his game has been one of the most memorable moments in his life. He was blown away seeing Bruce conducting the orchestra live.
So the score was recorded in 1995 (we finally now that for sure, the "1990" in Bruce Broughton's notes in the CD booklet was a typing mistake), the game was eventually released in 1998 and Intrada happily came 1 year later (thank god they did that!) to release the score on CD.
It took 6 years to make this game, it took me 6 years since my initial post (and 15 years since the release of the CD) to get my answer !
