If the consultancy delivers vulnerable software to the customer, the customer would most likely sue the consultancy for any damages they incurred. Especially if there is a maintenance contract, where the consultancy handles any security upgrades etc that are necessary.

Those are all good questions, I think the customers' perception is that they have outsourced this but obviously they too have a responsibility (for instance: to do due diligence on the parties they do business with to make sure that they are observing their duties as controllers). One thing is certain: the data subjects are without blame and their interests have been violated substantially, the question really is what else happened to the data and that is something that nobody knows right now. But if it is true that it ended up on common crawl then that would be quite a big problem.

The controller/processor relationship is very clearly described as part of the GDPR and there is little doubt in my mind that both of these parties jointly and separately carry responsibility here.