I think we can all agree that Sabo wouldn’t die off-screen, so my guess is that Sabo got captured and is set to be publicly executed.
You will remember that when Ace was going to be executed, the World Government wanted to end Gol D. Roger’s bloodline but also wanted to lure Whitebeard to finally confront him in their own territory.
Here the situation is the same: they are going to use Sabo’s execution as the perfect opportunity to lure their undisputed number 1 enemy, the Revolutionary Army, to defeat them once and for all.
A popular theory is that Makino, Koala, etc… are crying because Sabo got framed for the death of Cobra but we see in the same chapter that the World Government is trying to cover up the incident by sending Morgans money and by also going to the extreme lenght of sending a member of Cipher Pol to make sure that the truth doesn’t spread out; if the no.2 of the Revolutionary Army got framed by the news for the death of Cobra, why would the World Government try to cover that up?
One of the representatives of the Revolutionary Army, the organisation that is trying to overthrow the World Government, is the one “responsible” for an innocent’s death and this innocent man was also the king of a nation; what better way to make the commonfolk go against the Revolutionary Army than this? Obviously this is not the case, and if it were, the World Government definetely wouldn’t try to cover it up.
No, the reason they don’t want the truth to be known is because they did something bad (trying to erase the Nefertari Family) and want to cover it up to preserve their image in front of the world’s eyes (just like they did with Ohara and the White Lead disease incident).
Now this is where my theory really starts, so buckle up. In every single line of dialogue the Revolutionary Army appears in chapter 956 they are always expressing concern in regards to Sabo.
Nobody mentions the other officers that were also there (Morley, Lindberg, Karasu), but they do mention that they can’t contact them to have confirmation on what happened for some reason or another, so most likely the newspaper only mentions Sabo and not the officers, implying that nothing major happened to them.
Vivi and the other officers of the Revolutionary Army managed to escape safely with Kuma while Sabo stayed behind; please give me the time to explain. Vivi is hunted by the World Government under the direct decision of the literal king of the world Im, so you can bet your ass that if they had/have the chance to kill her they will; but we can all assume that just like Sabo, a major character like Vivi (ex-Straw Hat) isn’t going to die off-screen.
So that’s what the “attempted murder” Morgans was talking about is. The only reason the Revolutonary Army is at the Reverie is to retrieve Kuma, their former comrade, so you can also bet your ass that there is no way that Oda brings almost all of the major characters of the Revolutionary Army for ONE reason and they don’t accomplish it, that would be extremely bad writing if all of them got defeated and they went there for nothing, and it’s just not what Oda does.
But how would a normal human such as Vivi and a soulless Kuma that doesn’t have a conscience anymore escape from 2 Admirals? Well, they escaped because of the other officers of the Revolutionary Army and their powers: Morley can dig tunnels underground with extreme ease because of his devil fruit ability and Karasu can fly while carrying people on his back as shown here in Dressrosa chapter 794:
And that’s why the news don’t mention the other officers of the Revolutionary Army: Sabo stayed behind to stall the 2 Admirals so that the others could complete the mission (saving Kuma), but was obviously defeated in the end.