As far as I know both NiM DG kills have been done with 1 Merc DPS in the group. I know one was pizza, but the other one wasn't. I agree with most else what you said, which makes me kinda sad because I love my commando and hate knowing that it's sorta subpar, and it's gonna be hard to justify bringing her to raids when I finish getting my gunslinger up and geared. Still, I just wanted to comment that yes indeed everything can be done with Commando/Merc DPS. Course you know that too I just wanted to emphasize the point =)
Dont level a Slinger if you want to play Commando, plain and simple. I too have a dusty Level 50 Slinger sitting around somewhere, but dont mention this to anyone
Also, given DiLiH's kill was done with a (no doubt very good) Vengence Jugg, it's not outside the realm of possibility that you could do the fight with 2 Commandos. Also, while it would be ridiculously suboptimal I don't see really why you couldn't do it with 4 very VERY good Commando DPS. Maybe even just pretty good commando DPS. They'd have to really know all the little tricks to upping their DPS of course.
Maybe, maybe not. I haven seen enough of Kelsara enrages with my own parses to have a real opinion about it. But the fact remains, other classes can do more with less skill and gear. No way around that. But thats no reason to beach your Commando, just take it as a challenge.