I think it is buggy or has been patched.
When I first tried to do "All The Pieces" on my main (2 weeks ago), I asked various guildies for help. Some had fully completed the quest chain; some hadn't started. All of them got the "not eligible" message at the platform. I then completed it after waiting for another guildies to get up to the same step.
Now on my alt, I was able to solo completed "All The Pieces" (yes a dps merc with mako can kite, heal and interrupt). Yesterday I met someone at Ilum to finish the seeker droid quest; he happened to be at the "All The Pieces" step; so I offered to help (hoping to find 2 more people to do Shroud's Last Stand after). To my surprise, I was able to get pass the platform. I got the "not eligible" message when I clicked; but then I got the "spectator mode" cut scene.
Maybe it is bugged, works sometimes, not others. My experience with the area happened this past Saturday.... The only way to get past it was for all 4 of us to reset the mission, which is not horrible, takes maybe 10 min to re scan the 3 locations. I know some of the members in the guild I am in will want my help, so if I can help, it would be nice. The other 2 guys we did this with are in a much larger guild and one is the GM, so he was super disappointed thinking he will not be able to help his members.
You soloed that boss fight? I do not think I would even try that, I would still want my healer (my wife)....