Yes, Lille will benefit from playing with their fans' support. The situation is still difficult to predict and will depend on the team's readiness and the strategy used by both coaches.
Dortmund or Lille still have a chance to win. It's just that there is a possibility that the match will be decided by extra time or even a penalty shootout. When both teams cannot win in normal time, then the next thing is about their mentality.
I don't see the chance of the game ending in a draw for the second leg and I am sure both Lille and Dortmund will try their best to finish the match in normal time. The situation is indeed quite advantageous for Lille, but that does not mean Dortmund will let them play more openly to beat them easily. The coach's strategy will determine the steps of one of the two to qualify for the next round and I am sure there will be a surprise later. The question of mentality may be determined based on experience and in this case Dortmund will be much better but there is no greater guarantee for them to win the match.