I've been thinking about a comprehensive solution to this problem, and I think we need to deal. Ami did so, in a similar situation with much less effective leverage, and I'm inclined to trust her judgement on this.
Fundamentally, Orochimaru appears to be rational and pragmatic. Even though he puts up this whole pretense of being an unstable, murderous S-ranker, we know from observing his broader choices that he can absolutely function in the social context of a Hidden Village, if mostly be removing himself from it. He gives no real indication of enjoying bullying people - it's simply the most straightforward way of getting what he wants. He is far from stupid though, and being too forceful about this kind of thing is what got him labelled a missing-nin in the first place. Given that he returned when given the chance, and that he follows Asuma's orders, he places significant value on being a Leaf ninja.
As such, I don't even think anything immediately bad would happen to Hazou even if Mari hadn't interfered. If Orochimaru kidnaps a Clan Head in broad daylight, his operation will go up in flames. I suppose if he *really* wanted the Iron Nerve, he could make that trade, but I doubt we're close to that. It is, however, very advantageous for him that we believe he would, which is why he keeps playing into that stereotype. If anything, I'd expect him to try and obtain a different Kurosawa before making as huge a mess as killing Hazou would produce. Think about why we don't see Oro waltzing into the Uchiha compound to kidnap Sasuke, very similar reasons apply to us.
With that said, he can absolutely make life difficult in the medium and long term if he decides he really wants something from us. The only way to get around that is to make him a deal where our cooperation provides better outcomes than conflict. Aside from making the carrot of that arrangement enticing enough, we need a big enough stick to phase him. Just making clear that we're not going to just fold if he oversteps a boundary should be enough, I think - we can absolutely escalate from getting Asuma involved, to actually bombing his estate down to the 10th level, if pushed.
I actually suspect Oro might treat us better after we stand up to him. His treatment of Hazou in particular is a bit strange and incongruous with the rest of his behaviour. My suspicion is that he actually has some expectations of Hazou, but that we won't see any benefit of that until we can earn some of his respect - which is impossible if we keep being a doormat he can step on with impunity.
I am aware this is a risky move to make based on some amount of speculation, but I can't see any alternative. We either lean into our leverage and stand our ground, or run and leave Leaf behind.