I mean, where can you go from here? I've been trying to organize this mess, and think of a possible solution without having to retcon anything, and still have it make at least some semblance of sense, but I just can't. There's just so many gambit pile-ups, contrivances, plotholes, pointless and yet somehow plot-relevant characters, world building consistencies, continuity errors, and broken powers that I can't even begin to think of a way out of this.
Given the circumstances, the Marleyans winning is the only ending that makes sense at this point. Anything else would just be bullshit. So naturally that's what's going to happen.
Ok, if this chapter is false and I get to retcon shit. Here's where he could, or where I would if I was suddenly put in charge of this mess. This chapter is what the future WOULD have looked like if Eren and Zeke hadn't met. It'd 100% be time fuckery on my part. I'd start it with basically Eren looking at himself die. In reality, he's already met Zeke already and this is playing out like a future that didn't happen. His power over the Coordinate has made him master time and space itself. The next chapter would be exposition. The girl resurrecting Zeke is the 'good' portion of Ymir. Ymir's personality shattered when she made a deal with the devil, trapping part of her soul where all the coordinates meet. This part of Ymir's soul is trying to mend the world that Yimr broke. Then the rest of the manga is Eren going through time and space, fighting Satan and Ymir, deposing of the status quo, eliminating Titans altogether and irreversibly altering the future and the universe itself. Eventually he succeeds, perhaps destroying the part of Ymir's soul that bargained with the devil, eliminating Titans wholly, changing the entire course and history of the world. Then we go through time and space, everything re-winding and altered.
The Titans don't exist, hence Marley and Paradis never exist, there are no Nazis, no concentration camps, the world is a fundamentally different place. Eren is reborn and Grisha is his father, but in a completely different universe where none of this strife ever existed. However, he never meets Mikasa, Armin or any of his former friends because the universe is fundamentally altered completely. Its a 'bittersweet' ending in the fact that there is no more living hell, no more suffering and everyone is truly free, but he will never have the relationships we've seen him have before. Eren will be a completely different person, so will Mikasa and Armin, but they'll never know the pain and suffering of living within those walls. Its a 'happy' ending because there's no more horrible man eating monsters, everyone who died during the series is alive but different people. Ackermans are just normies now. Eren has saved countless lives, ended untold suffering, but no one will ever know about it, not even he will since he is reborn as a child and the course of the world is entirely different. I know I'm basically resetting the entire universe, but I just don't see a satisfying ending if you have Gabi kill Eren and everything just fails and be pointless. Like, I don't need a story to tell me sometimes things end in utter failure. That's just wasting my time.
That's really one of the only ways of saving this mess with a 'satisfying' outcome.
If the chapter is true, not mind magic or time fuckery, but actually Eren getting his head blown off...I really don't know. We're going to assume we're looking for a satisfying outcome for the reader. So I will make that assumption by going on NOT retconning this chapter. Eren's head gets blown off, Zeke is picked apart by Gabi. Just as he's about to die, he touches Eren's head, whose consciousness remains in there long enough. Then this happens:
Eren and Zeke's minds meet up. Zeke wants to carry out the plan to euthanize all the Eldians. Eren has none of this and they both fight in their mind for power over the Founding Titan. Eren wins and enables the full power of the founding Titan, Zeke's consciousness is utterly obliterated. Eren's body reforms itself through the ultimate power of PATHS, as he can now control time and space and is remade basically from the sand where Gabi blew his head off. Basically we're going to massive power levels here. Eren himself forcibly stops the battle, except for Mikasa, who is an Ackerman. There, he explains the way he was acting because he saw that there was only one path to peace. He had to ally with Zeke and manipulate this cult of followers so he could get close enough to subsume the founding Titan's will. He needed Marley to come to Paradis with all of the Titans in one place, so he attacked Liberio knowing they'd retaliate with everything they had. In order for them to be free from war, he needed to subsume the founding Titan's will and demonstrate its power. The girl in the PATHS is the good part of Ymir's soul. Having the ultimate power of the founding Titan, he returns all the Titan's back to normal, including the shifters. Because he can freely move through time and space, he realizes if he resets the universe, it results in an infinite loop that can never be broken. He's done this before. So he decides to rip apart all the powers of the Titan shifters, merging the 9 Titans in himself, ripping their abilities into himself using PATHS. He announces there will no longer be any Titans, Titan shifters or even the power of the Titans. He will be the last.
Using the power of the founding Titan and the help of Ymir, he shears apart the very PATHS themselves, eliminating the link between descendants, Ackermans, everything. No Titan power can be passed on. The walls themselves just crumble into dust, because the Titans are all destroyed. Anyone who turned into a Titan reverts back into human form. Annie comes out of her crystal. Eren will live a full life because he's eliminated Ymir's curse. There are no more warriors, no more nothing. To continue fighting would be futile. The threat and terror of the titans has ended. Eren is the last Titan. He demands the fighting stop. He won't manipulate them or alter their memories, but he's basically an unkillable God right now who has torn apart time and space. That alone basically forces a parley. Then we have an epilogue and whatever.
This is still asspull central, but fuck it.
Alternatively, without asspulls, Eren dies and Marley wins. Or Eren dies and the manga continues without him somehow. Which is completely unsatisfying. So I have no fucking clue.
For me the most likely option is 2 only because Iseyama is a massive autist about continuity considering he plans on bringing back things in the manga around the same time the anime introduces them (i.e. Brining Keith back around the same time the Bystander episode came for the anime). Iseyama is a hack but one thing to give him credit for is that he is a huge autist in regards to how continuity is.
Who knows man. Its just really bad no matter which way you slice it.