Salzburg can still be targeted by Real Madrid for 3 important points that will help them maintain their automatic qualifying position. If they win over Salzburg, then Real Madrid's points will be 12 which will be able to maintain their survival even though they have to fight through the playoff zone.
In order to qualify for the next round even though there are two more matches left, I feel that they can qualify for the last 16 even though Brest is an opponent that can be considered tough considering their achievements during their six matches in the Champions League.
Real Madrid are only 3 points away from the top 8, and a win in two matches could change the table a lot, but on the other hand, those teams that are in the top also won't want to lose their positions. Maybe even if they can't get into the top 8, I have no doubt that they will make it to the playoffs. The difficulty will just be that they might get a tougher opponent in the early stages.
One win and one draw is enough for Real Madrid to advance to the next round with the scenario that other teams also want to win.
My worst case scenario, the home game will be won and the away game will be drawn. If it really happens, then 4 points will be obtained and can save Real Madrid's hopes for the next round.
It could also be that in the last two matches, both of them Real Madrid can win.