Yes. But the aim of building this one was to go look at the Titanic. I think it is fair to assume that the designers had a specification in mind that they thought would be sufficient for that job.
Right. But who were the designers?
If a naval engineer designed it for 3900 meters, and certifed it as such, then he's liable. But if OceanGate had the submarine built to print (which means they gave someone the design and said "build this and I will pay you") then that someone is not liable for any design flaws in the design. Sure, if they spec'd a specific standard for a weld and they did not make the weld to that standard, then they are liable. But that is VERY difficult to prove due to the catastrophic nature of the failure, and a sufficient defense is that the design was not expected to survive anyway.
Reading about Stockton Rush and his approach, I strongly suspect that something like that occurred. For example, the viewport the sub used was not spec'd to the pressures it would have to withstand. One of his workers, underwater inspection specialist David Lochridge, was fired and sued when he expressed concerns that the submarine had not been subject to NDT, a very common inspection methodology. Rush has stated that
“At some point, safety just is pure waste. I mean, if you just want to be safe, don’t get out of bed. Don’t get in your car. Don’t do anything.” He has also stated that "I think it was General MacArthur who said you’re remembered for the rules you break. And I’ve broken some rules to make this. I think I’ve broken them with logic and good engineering behind me.” That suggests a disregard for basic design verification.
If that is the case, then the person who will be ultimately held responsible is now dead.
That won't matter, if negligence can be established in a court.
It actually does matter. In a landmark case (Hulsey vs Elsinore) a student skydiver sued a dropzone because he became injured due to an instructor's negligence. The case was thrown out on the grounds that they had signed a very explicit waiver, stating that skydiving was in no way a safe activity, and they risked injury or death if they tried it - and that they did not warranty the competence of their instructors or the safety of their gear. What was notable about this case is that the decision was not based on whether the instructor was negligent or not - the case was thrown out because since the student signed the waiver, they no longer had a right to sue due to negligence.
Needless to say lawyers can try to sue for anything, and they will no doubt try in this case.