Narcissa's no slouch. I don't think she's a match for Bella, though. Not in a direct duel. When she says she would have acted, she means that she would have made Rod and Druella and others aware of the ways that Bella was treating her children, and would have either found a way to convince Bella to give up her custody or have got someone else to stand with her and do it.

I don't know if she would have succeeded, but the other thing about that is that up until the time she wanted to secure the Manor, she was supremely confident that Bella would not actually try to kill her, so long as she did not actually try to kill Bella.

As for why she chose not to be a wand-wielding kickass...? They had enough of those. Her interests lay elsewhere, and her talents lent themselves to assisting Lucius in his role so that together they would be the Protectorate's First Couple (second only to the LP himself).

I will let Molly and Bella's players speak to that, but IMO while that moment was a triumphant note in canon, it's also a problematic one. Molly uses the Killing Curse and we're supposed to cheer, for one thing. But beyond that, I think Molly had the advantage of surprise in that instance. Then again I really only read DH once....