>What the fuck happened
You said it yourself. Trump happened. Marshall has become the Stan to Don's Em, and he absolutely can't stand that idea. El Presidente has made him his bitch.
>Why is his flow so choppy now
Age, stress and drugs. I don't even think he can do "fast" anymore. As it stands, he's just above the '06/'07 slurring zombie era.
>Why does he keep following trends instead of doing what made him popular in the first place?
Because he tried that, with Relapse. And all the under 12s who shouldn't have been listening to his shit over Lil Wayne or w/e the flavor-of-the-month nigger was back in 2009 and over 21s who should have moved on from him expecting a TES 2 thought it was crap. Because of them we were robbed of a Relapse 2 and ultimately sent Em down the direction he ended up taking.
>Why does he neglect to put on his past associates? (50, Dre, Xzibit, D12, etc)
You'll have to ask him himself. It seems to be a strictly Eminem thing, and his record label is where up-and-coming talent went to die. Obie Trice, Bobby Creekwater, Ca$his - who even remembers half of these guys? He was (and thanks to the Slaughterhouse debacle, still is) worse than WWE at burying people who could have been stars.
>Why does he seem so out of touch with what people want from him?
Because he is out of touch. The man can play a genuinely mean game of Donkey Kong (seriously, look it up, he was setting near WR-scores a few years ago) but is absolutely CLUELESS on how to operate a computer.
>Why does he say his music isn't as good as it used to be, then get angry when we agree with him?
No desire to improve, he feels he's done it all at this point and it shows. Sometimes I wonder how MMLP2 would have fared if he'd had the patience to finish it off as a double album instead of the 1 CD + strangely long bonus disc it ended up being. Most of the tracks on that second disc, barring Desperation, were actually better than a lot of the ones on the first. Wicked Ways should have been the final track due to the Ken Kaniff skit.
Personally, if you want someone to blame, blame his younger brother Nate. The little shit leaked half of the decent tracks from Encore a year before the album even came out on shitty cassette recordings, forcing him to put what otherwise would have definitely been rejected or relegated to the B-Side onto the main album. Encore bombing is what led to Curtain Call and his hiatus which was set in stone once Proof died.