In answering the question, who is most at fault, as opposed to solely or entirely, it's Spalletti, unquestionably.
Spalletti is the one who is meant to train and coach the team in terms of actual football on the pitch. Yet we see little improvement, if any, in our style of play. Inb4 some fucking cunt (probably brehme) starts talking shit about 'playing beautifully' or some trash, the team has looked largely clueless on the pitch and 'beautiful' or 'entertaining' football has nothing to do with it. On the contrary, unsustainable, non-developing and completely ineffective football is the problem. The players clearly lack any semblance of a structure or framework to follow on the pitch. That is, outside of 'spread the ball out wide and cross it to Icardi who is alone in the box'. That's on Spalletti. Marotta, Zhang and Ausilio don't coach the team. Icardi and Perisic do not coach the team. Spalletti does. Even a relegation side has less trouble creating clear cut chances than us. Even a relegation side plays with more cohesion and goal-oriented functionality than our team.
In crucial moments where his tactics have worked to a point, Spalletti is again the culprit for some unexpected and unnecessary defeats - defeats which (in the manner they happened) are sure to cause damage to the morale of the squad. I would argue we have paid the price for Spalletti's failure with tactical and in-game management more than we really should have. Whether we're talking about the now infamous Santon - Icardi sub vs juve last season, or the Valero - Politano sub also vs juve this season, or trying to "outwit" Mazzarri by cloning his 343, or going to Tottenham with zero intent of winning, or the Vrsaljko - Politano sub and instructing the team to waste time against PSV etc, there are several clear and damning incidents where Spalletti has directly ruined his teams chances. Far too many.
A lot of the players are turds and let's not forget some of them are direct and specific requests from Spalletti. No doubt the management is restricted by FFP and the SA but should've been more diligent in finding usable cheap options, even more Cedric's purely to provide some more options. But the one who takes all these pieces and puts them together is Spalletti. So far he hasn't managed to find a dependable or sustainable system on the pitch. He has also compromised the present and future of his own work with his frequent self-sabotaging moves. He is responsible for a lot of good too, but for me he is the standout guilty party for 'most' at fault for our current situation.