agreed - I didn't acquire this quest until after bumping off the Thorian, but like yourself was unable to do it because the elevator to the skyway no longer operates.
IF this quest is ONLY possible to do if collected as a quest BEFORE going across the skyway, then it should no longer be offered as a quest (which can never be completed) AFTER the main quest is done.
The answer is for the elevator to not be locked after doing the main quest.
There is a 3rd party mod on Nexus which unlocks the elevator to enable skyway access so you can go back and do this, but not everyone wants to install mods.
Despite apologists who say you should just pick up the quest before going across the skyway so it therefore is not a bug, I consider that it IS a bug - it's a breakdown in logic. The bug is that you are even offered this quest (which is now impossible) following the main quest completion.
The simplest bug fix is not to lock the skyway elevator.
