2) It's the same limbo. Based on Saito's look, it's been 'decades' since Cobb set out to find him, and actually found him. In that time, Cobb's memory of how he ended up there has faded. Meanwhile, Saito's been architecting his immediate environment (and that's all we see, we don't see anything else in the distance and if we did, my guess is that it would be the same 'skyline' Cobb and Mal architected).

3) The sedation is the difference. Cobb and Mal weren't sedated when they were in limbo, so killing themselves worked to wake them up. In this case, Fischer's under heavy sedation and dying wouldn't wake him up, even from limbo. Hence, the defibrillator was used to 'kick' him up from limbo to level 3.