Oda is already preparing another major problem for the SHPs and Vegapunk. Because, if they want to flee, it is Stussy, who has to deactivate the Dome, whenever the group is ready to depart. This is a crucial task, as it is solely on her to push the button for the Dome’s deactivation. No one else is currently around to push that button in case she would not do so.
And I told before that Stussy is not on the SHP’s side, nor is she on Vegapunk’s. She is with the WG and therefore will do anything necessary to prevent Vegapunk from fleeing. Someone has to interfere and stop her from doing so, if the SHPs want to be able to flee.
The conversation with Kaku therefore is a little awkward and already hints at her betrayal. Her words to Kaku’s question of, if the SHP’s left her in the control room, was: “Unfortunately, I decided to stay on my own. Someone has to take on the responsibility for this final mission.”
Notice how she says “[…] for this final mission.”. This sentence only vaguely states that she has to do a mission – but not what this mission entails. Why would Oda not directly let her state that her mission was to make sure the Dome came down when the SHP’s are ready to flee? Why use this vague phrasing instead?
Using such vague statement would only make sense plot-wise, if Stussy’s mission was not to deactivate the Dome, but to make sure the SHP’s and especially Vegapunk were not able to flee. This way Oda could keep it a secret that Stussy is on the WG’s side, without us noticing anything – only by keeping Stussy’s statement as vague as it is.
By the way, we still do not know if Vegapunk died and if his death made the transponder-snail start playing the pre-recorded message. We only have the elders stating that they assume it is this way, without having a verification of it in any way. So, there are still two possibilities: Vegapunk is dead and Vegapunk is still alive.
To the question of what Mars saw at the end of the chapter: It can be quite obvious, but I will just dance around the answer, as there is still a possibility that it is not the most plausible answer. At the beginning of the arc, Vegapunk revealed that his brain is actually in Punk Records. And in chapter 1075, Robin and her group are coming about a floor, where Vegaounk grows organelles, which are swimming in tanks filled with liquid. The sfx Mars hears tells us that there is a liquid bubbling. And if there is the Stella’s brain up in Punk records, than it has to be stored somehow. And the tanks, filled with liquid would make more sense than anything else as of now.