Focusing on the UCL and ignoring the Bundesliga was a big mistake made by Dortmund, even though they could beat Lille in the round of 16, they will most likely face Barcelona in the next round. Dortmund must take a more realistic look at their abilities including the chances in the UCL which leaves strong teams from different Leagues.
Dortmund must maintain their position in the Bundesliga in order to finish in the European zone in anticipation of failing in the UCL.
If you remember in UCL last season, maybe that's the reason why Dortmund is too ambitious in UCL and seems to focus on UCL. But I still believe that even though they are currently in the top 10, Dortmund always has another plan to still get results that are not too disappointing.
As you said even if Dortmund can qualify from the round of 16 and fail to face Barcelona, they still have a chance to catch up to the top 5 or even the top 4. Because Dortmund only has a 5 point difference to get a UEL ticket.