Why does MC have to get back at her? They are discussing other matters, and Malsara knows she showed herself a fool there, so she just throws back an insult. But then again, it's up to how you interpret social clues. Someone would go 'HA she had the last word therefore she won' and someone would go 'they both know she just admitted she is not as good as she claimed and tried to insult MC to feel better about herself like a child'.
Who do you want to intimidate exactly? I explained why it wouldn't work with the servant: you'd probably need to get physical, and then it'll lead to MC getting their ahh whooped by the guards or smth. The politicians, the count, the mages — none of them are scared of MC because they all are firmly in a position of power. Once again, short of drawing a sword at them (which will be a dead end), you won't scare them. And if MC were to try to threaten them, well, then they'd make a fool of themselves, and you'll be there saying that MC is spineless because no one fears them

IIRC, you can double-threaten that merchant guy in Rona, the mother (though I only tried it in the pre-rewrite version, so don't remember if it stayed in the newer one), the lady that took the chalice, the bandits, and the punk that tried to rob you, and, not sure if this one was left in the current version, the mage apprentice in Rona. Basically, people you can threaten with brute force or by just being big and looking scary. Which tracks because MC has no positive or negative rep, but they are a magiani, so... kind of a freak in their eyes. Overall, circling back to your starting thesis, I don't think the MC is spineless, I think they are just written in a way you wouldn't play them (which is valid; I don't play PoMA or DAO Ascension for exactly same reasons, and I like Wayfarer because it gets about 80-85% of what I want my MC to act in this situation, realistically.)