Mikel Arteta was able to show this improvement because the owners actually trusted him with their money. I remember there were a lot of big changes made in the Arsenal team before this season started. All those money that they spent are actually making sense right now.
Arsenal did bet risk with Arteta but they somewhat had to do this too. They did not have good position to attract good coaches to their club after many years of failure and after a failed choice with Unai Emery, they turned to use a young coach who used to be their player and understood about their club philosophy. There was some early bad months but so far so good. Both Arteta and Arsenal have gone through two years with each other very well and they are aiming at their first trophy by beating Manchester City in a race. After many years, it is a first time Arsenal can actually "smell" the trophy very closely like this.
It was really frustrating for Arsenal to lose the Champions League spot in the last match of the last season. That's why I believe they went all in.
Losing a spot in top four for Champions League qualification is impossible, Arsenal won't lose it with only last 10 matches of Premier League.
Arsenal lost in the Europa League match this season but it was still very close. But I don't think Europa League was the target for Arsenal in the first place. The main target for arsenal was the English Premier League title. And they are doing a very good job trying to get it.
A shocked defeat in Europa League at their home stadium is bad but it is good for them generally. They are no longer have to decentralize their limited resources for two competitions. After that, only one competition that is Premier Leauge and they are fighting for the crown, not for lower position.