The red is what will never make this beyond a flawed idea. Take this out and just make this an advance-advance / Alpha Class idea.
Although the story I consider beyond non-canon, The Force Unleashed Ultimate Sith Edition, Obi-wan's force ghost is able to attack you wiht powerful force attacks, in Star Wars lore force ghosts of wraiths have been known to violently attack other beings, etc.
Your idea to take away gear and combat would be the death of TOR because it makes no sense. If anything this "Force Ghost" would the the ultimate healer or dps. The "retirement" aspect wouldn't be something to look forward to to be hoenst. It would make one feel like "wow I did all this work...and I can do jack **** with this character now but X and Y ....might as well stop playing on this one...."
I like the buff idea, but in general this class would be beyond OP in PvP most likely. IF TOR had a duel skill tree for PvE and PvP I'd be for this 100%, but for the moment, if you tweak this idea more realistically I'd be for it.
I guess I was thinking that if the group crashes, the group crashes, seperate from the Force Ghost, who is not engaged in combat per say, though he is a part of the group. He cannot be the last one standing against a Boss, he cannot complete objectives in PvP.
I think my idea is largely influenced by LOTRO where there classes whose primary role was buffing and debuffing. It's not as if the Force Ghost would just sit there. You can keep them very active by casting constantly, with buffs/debuffs with very short cooldowns.
So my thought would be that you would still be engaged in "combat" but a modified version.
I think your'e right about the gear/progression. It would be necessary to have some continued development. Again here I'm a bit influenced by LOTRO. You hit max level and development takes a whole new direction, you're no longer trying to level, but enhance a particular "legendary weapon." True, you also still get gear. I'm just thinking of the process dynamic. So in the case of the Froce Ghost, you may not go after a "chestpiece" but learn a deeper "mystery of the Force" to improve your debuff or something like that.
Thanks for the suggestion and being open to the idea!