I think part of the problem with the overall story is that they made the problem "too big".
I would say the problem is that the exact requirements are not clearly established, such as exact number that need moving, precise details about available ships, what actions the Romulans had already taken to move the population etc etc.
Or was it a case of every Romulan for themselves with some escaping easily leaving others to die.
In the context of the show its not really required though, the exact details dont really matter all we need to know is that the Romulans knew they could not (or would not) do it themselves and finally looked to the Federation for help.
I can definitely understand why some would not want that help to succeed as its a move out of the Romulans own playbook, saving the Romulans could have been a big step to reunification or a true alliance between the Federation and the Romulan Star Empire.
Ultimately its the Romulans themselves who would stand to gain or lose the most from such a move depending on their goals and motivations.
It still doesn't explain the timing though, if the Zhat Vash were only interested in destroying the synths they could have attacked Mars after the rescue had been completed or was underway, some other external event must have triggered it just at the wrong moment, perhaps it has something to do with that Romulan ship that was attacked by the Borg as that happened at roughly the same time.
I am not convinced it was the Zhat Vash that reprogrammed the synths, we know they hate all forms of artificial life and would just destroy them rather than make them go mad, it is a smart move using the synths to do it though as it would not be traceable back to themselves.
The best way to identify the true culprits is to consider the overall motive behind the attack, was it a warning on the dangers of over reliance on synths, if so that would fit the Zhat Vash but the timing is just all kinds of wrong.
Was the attack triggered by elements of the Romulan military who were resentful of others asking for the Federations help, I cant see it myself as it would require a lot of time, planning and resources to compromise so many synths to ensure a successful attack with no survivors on either side.
Could the motive simply be down to having the means and opportunity to weaken Starfleet due to having so many ships and resources in one place, with the added bonus of knocking out Utopia Planetia as well.
Perhaps it was orchestrated by elements with the Federation/Starfleet who really didnt want to help the Romulans, we saw something similar in the Undiscovered Country with Valeris conspiring with General Chang.
We cant really answer those questions until we can ascertain whether the Supernova and the attack on Mars were connected and triggered by the same organisation/group, if so the attack on mars was simply a means to ensure that the Federation did not intervene on the Romulans behalf in which case it could not be the Zhat Vash as they dont have the means to destabilise a Star in that way, if the two events are not connected then the Zhat Vash could be the culprit of the attack on Mars but again why would they scupper their own salvation.
I think we will only truly understand events when we find out exactly what happened in the distant past, it must have involved the Vulcans as well as the show seems to indicate it was before the Romulans left.
