Although, we actually don't know how long the Jellyfish was being refitted for. It could have been weeks (it looks like they stripped it down to the skeleton in the mind-meld), giving Spock ample time to head back in order to fly the mission himself.
So they couldn't have had a starship go to Vulcan then onto Romulus in the same time period? The NX-01 could get from Earth to Kronos on four days at the old scale Warp 5.