I can only speculate as to why they did it, but I am pretty sure that it was to get the whole process started because some of the heirs were in poor health and they needed to sign the contract sooner rather than later, otherwise they would have entered in a potential loop of heirs dying and having to find the heirs of the heir and so on. So they said, let's start this thing now to minimize that risk and work on the rest of the documents later. From this point of view, they were right, as two people died in the months that followed.
I am also sure that they've hidden the fact because they knew that in this situation, it would have been nearly impossible to find a buyer willing to take part in the risk.
You are right with your remarks about experienced buyers. All in all, I have almost 0 regrets about the whole thing, I learned a lot of stuff in the meantime and got the house eventually
They already gained something - the sale of the property.
> knew that the remaining coinheritors would block
This doesn't matter - they said it was done. This was incorrect. It was fraud...
Perhaps a more experienced buyer would have added some penalty clause to hedge against the idle money issue (banks don't like a delay between money availability and purchase of the collateral, that's a common issue I think), but since the problem is (was, I hope?) that the contract itself did not happen in time that surely wouldn't be easy (some multi-stage monstrosity, ouch). Unless they lied to get some advantage it can't really be fraud. (and what advantage would it be anyways? If the market price rose during the delay the sellers might even suffer a bigger net loss than that bank penalty ... I guess that would be a way it could become fraud, if they then suddenly refused signing, to sell to a higher bidder, abusing the delay for a safe market bet)
I suspect that this entire class of issues is a common pitfall people who routinely deal with real estate are well aware of, one of the risks kept in check by experience and taking the occasional blow.