Based on the information you've shared it seems as if the damage was minimal and I doubt it' would be worth the expense to cut open ceilings.
It's possible you could use something like an ice pick to poke through the ceiling and prod the floor joists above to see if they feel at all damaged, though this isn't fool proof. I would think your home inspector would likely have seen some other signs indication any significant structural damage as well.
Were it me I'd be satisfied, but if you're at all concerned I would open up the ceilings and assure yourself. Speak with your broker perhaps you can negotiate splitting the cost with the Sellers, though this might require you to threaten to terminate your contract.
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