About the Copa Del Rey, Real Madrid has one goal of disadvantage, but if they manage to score at least two goals it will turn out to be and advantage because they will win the match even if they draw, as away goals are worth more (correct me if I'm wrong).
Away goal rules no longer exist in the copa del rey competition, you can check it in the internet to find out, i think it it was removed from the copa del rey competition 2 years ago, so to qualify a team must win the tie, because a draw cannot help any team to qualify. Real madrid have to beat Barcelona in the camp nou to be able to qualify for the final, as any type of draw in the camp nou and Barcelona will qualify, because they won the first leg 1-0. I believe Barcelona will go to the final, they will either win the second leg or draw the game.
It will be a tough match for Real Madrid to qualify for the finals, they need 2 goals to ensure themselves going to the finals. But if they can't, then even scoring 1 goal is enough to go to 30 minutes of extra time. At Camp Nou Barcelona will have the advantage because they will have the full support of their supporters, while Real Madrid will have to fight even harder after they failed to win the game at home in the first leg.
The opportunity to win La Liga this season seems to have started to close for Madrid, because in order to catch 9 points from Barcelona, Madrid must be able to win from the opposing team while hoping that Barcelona will lose 3 times and obviously it will be very difficult, apart from against Madrid and Atletico in the upcoming matches the opponent faced by Barcelona is quite easy, even if madrid can beat barcelona in the 26th week later it looks like they will still be 6 points behind barcelona, as you said, it's ridiculous for madrid to hope barcelona will slip because however now barcelona is starting to focus fully on La Liga after they were knocked out of the Europa League.
Apart from that they have to expect Barcelona to slip and lose a lot, they also have to always win matches, whereas now Real Madrid often lose their matches, whether it's a draw or defeat. Obviously this is a very tough task to catch up with them. I don't see this as something ridiculous, because in football it could happen, but I prefer to say it's a very slim chance.