Madrid have risen enough from the downturn it seems they can grow that fast and overtake Barcelona, now their point difference is 4 points and that is too close for Barcelona to be at the top in a safe position, yes if they lose one or draw maybe Madrid can catch up quickly, this difference can still be chased easily I am also sure of that.
This injury may slightly hamper how Mbappe gets goals to compete with Lewandowski, but if Lewandowski is more productive in the next 3 weeks, I think at most he gets 5 goals in Mbappe's absence.
I do not think that they did anything different since the start, they got two draws at the start because of many many missing players, they had like 5 or 6 starters missing on those games, which is unusual because even if you have injured players, usually it is not that much from starters, so they had ties, because they weren't ready. They are still missing a few now, but most are back, and they adjusted eventually so they are doing fine right now.
I think it's clear that we are going to see Real Madrid win against Athletico Madrid as well, the gap between two teams quality is too high and no matter how much ATM tries to win, the only way they could ever win would be if Real plays worse than they normally do, that's it.