The first leg has already given a picture of how the second leg will be because we shouldn't expect any comeback from Real Madrid even if they will win Arsenal but the goals won't reach 3 and even if it does, Arsenal will also score one or two goals so the chances of a comeback is not possible for Real Madrid. I think the reason why Arsenal couldn't win their Premier League match today was because they were preparing ahead of the second leg hence they didn't even care about the match against Brentford. Arsenal is a difficult team for Real Madrid and even in their head to head. No matter how strong a team is, there are teams they meet and find it difficult to have an easy win and it appears like Arsenal is one of the teams that Real Madrid finds uneasy.
I agree with you that Arsenal’s focus is more in the champions league than it is in the premier league. We have seen them played poorly in some of their games in the premier league and one would have looked down on them not to have any chance against Real Madrid in the champions league but it turns out not to be so. The second leg will be full of lots of interesting moments but a comeback for Real Madrid is still what is not yet certain too much for anyone to say after seeing how Arsenal outplayed them and won the match in the first leg. Can Real Madrid do a comeback as they usually do in the past, that becomes a very big question of thoughts for one to ponder about.
If it wasn't a team like Arsenal, i could have calculated a possible comeback for Real Madrid but Arsenal are not cowards because even in the first leg many people underrated Arsenal so much but they proved those that belittled them wrong. The second leg will be tough since Real Madrid will now be playing at their home with their fans chanting support for them but the highest they can do is just get a normal win but the 3 goals Arsenal scored at Emirate stadium is enough to demoralize Real Madrid completely. Real Madrid have done some great comebacks even in the Champions League because i could remember some comeback they got last season's Champions League but at this point, Arsenal looks more prepared and dangerous than Real Madrid since the Champions League games started. Let us not forget that Arsenal gained a direct qualification while Real Madrid came through a playoff after winning Manchester City in the playoff round. As for your question if Real Madrid can get a comeback from Arsenal, the answer is capital NO.