Ultimately I think the common issue across the entire period is squad disharmony.
For me, the first outward embodiment of that was the mirallas/Baines penalty debacle but you can probably trace it's roots back to the permanent signing of lukaku.
In a cruel twist for us, the initial loosening of our purse strings and then subsequent investment appears to have done for us!
When Rom signed his wages broke our ceiling and if I remember rightly, Barkley got a similar contract within weeks. Not much later Stones got a hefty new contract as well. From a commercial point of view it made sense for the club to the down three young potential superstars to long lucrative contracts asap but that has got to have caused friction with others in the squad who were on deals signed before we upped our budget. For example, McCarthy had been excellent in his first season but the next year he found his young midfield partner being paid 3 or 4 times his reported wage. Can't be a coincidence he went off sick not long after. Was Distin put out that Stones was probably being paid more than him after a handful of appearances?
Since Moshiri arrived we are in the type of conundrum the likes of Newcastle have had for years. We have more money, but not as much as the top clubs and we don't have the success on the pitch to draw the elite players. We are constantly trying to show off but cant really back it up. So we end up paying too much for slightly better players than we already have and taking our eye off the ball in assessing players characters at the same time. If you have no money you have to scrutinise these things a lot harder and you don't take gambles. If you have loads of money then it doesn't matter as you can just replace/offload/loan out a player who turns out to be rotten. We still have to sell them to replace them - hence our squad is now almost entirely youngsters, past its or mercenaries only here for the pay cheque.
And sadly it shows on the pitch.