Nope. You're throwing points away for no reason.
I don't believe backstab works on guns. Melee only, last I checked.
But popping out of stealth with blinding strike and penetrating shots with a blunderbuss is hilariously effective on a lot of targets.
It does work with guns (and ranged weapons in general).
It works, but only within 2m. I would assume this is the source of confusion but tooltips does explain this. Also I think people are perhaps not fully accounting that stealth is time based. So you should line up your backstab from far away and make a beeline directly for the target, not position close slowly and then do it. You should be moving before you trigger the pie chart at your feet.
Lotta good info here. Haven't played the game yet, so tell me if this sounds feasible: Rogue in stealth beelines for a close target, opens with a blunderbuss backstab. Immediately turns around and runs away while tank(s) come up to intercept enemies. After tanks grab the enemies, Rogue swings back around to lay on ranged DPS. Workable or ticket to dead-rogue town? Probably depends on how many ranged opponents there are.