I think the tinkering was in part down to trying to accomodate Mkhititaryan and Ozil and not really knowing what defensive set up is best. He made tactical mistakes at the end of the season, but I think there were mitigating circumstances surrounding that. Hopefully he can get the players in that he wants this summer and create a stable defence that all know their positions and jobs.
I'm not going to lay into Iwobi though, I still think he's got a future at Arsenal. He's still only 22 and has shown enough promise to be given another season at least. When he's on the pitch he always creates dangerous situations, but his final ball lets him down. If he can improve on that then he could become a really potent player. The goal he scored in the EL final was brilliant. I'd like to see him shoot more, rather than trying to find the killer pass in the box.
Thing is, it's hard to criticise him too much for our defensive shortcomings because, statistically, we conceded the same amount of goals in the league as we did in Wenger's last season. But look at what we had.
A 33-year old, injury-plagued Koscielny.
A 33-year old left-back whose lost his pace in Monreal.
A 30-year old Sokratis who was new to the league.
A 35-year old right back who wasn't meant to play as much as he did in Lichtsteiner.
A Mustafi who should be in the prime years of his career, but was just as big a liability as he always has been.
23-year old Holding was in tremendous form, then gets a season-ending injury, which was a huge blow.
23-year old Bellerin, who was starting to come into form, then gets a season-ending injury too.
A 25-year old left-wing back in Kolasinac, who is decent going forwards, but not reliable at defending. He showed this last season too.
A 21-year old Maitland-Niles, who is a central midfielder by trade, but was forced to play as a right-back due to lack of options.
Even Simeone or Allegri wouldn't be able to tighten a defence with those problems/concerns.
However, I feel like our midfield at times didn't always give them enough protection. Xhaka is unreliable and is slow at making a decision or covering ground, but he distributes the ball well, and we had so few other options. Torreira looked burnt out after Christmas due to how often we played him. And Guendouzi, though looks like he can be a fine player, was too indisciplined in some games and wanted too long on the ball.
My only question is could Emery and the coaching team have done more to make us defensively sound with that personnel? I really am not sure.
RE: Iwobi - I would keep him at the club, but he needs to start stepping up pronto. He is 23 in May and the 2019-2020 campaign will be his 5th year with the first-team. His output for a wide player is really poor compared to those who play in the same positions at other clubs. He made 51 appearances for us last season too, so it's not like he didn't have the chances to show what he's all about.
If we don't see substantial improvement to his game in the final third next season, I don't think he will ever hold down a regular starting spot for us.