It needs to remain perfectly watertight at over 5000psi or everyone dies. That design constraint probably eliminates most clever latch designs.

I was imagining a round door in the middle of the hull, like a submarine hatch, but it looks like the entire front dome comes off and that’s the “hatch”. If so, can’t they just make the front a giant screw and add handles to both sides?

The dome shape is a structural necessity to withstand the pressure, like in a soda can. Else you would need more material, more material means more weight, more weight means more bouyancy required which again means more weight etc.

It's like designing a spacecraft but with different constraints.

It's pretty much the exact opposite of designing a spacecraft.

Then let me amend what I wrote: It's as difficult as designing a spacecraft, because you reach the limits of what materials that can be used can offer.

Well, once you are in space the forces are pretty tame until you get hit by something. But the ocean never rests from trying to kill you, even the smallest material defect can cause an implosion. Personally I think designing for a delta of 400 atmospheres in compression is much, much harder than for 1 atmosphere in tension.

But it's withstanding pressure(called external pressure, yesterday I learned), so can't they just have a clean, stepped flange to mate and to prevent slipping off, plus a simple lock to prevent unintended disengagement?

This is usually done with a plug hatch and it's not that complex. The Alvin submersible has one and it's from 1964.

Exactly. There are so many lessons learned already when designing a device like this there is no point in re-learning them the hard way, just adopt a tried and true design.

That's what they did? Alvin is an outlier; other bathyscaphes used the (tried-and-true) bolt-in method.

Assuming they're out there near the surface bouncing around in 6' waves and whitecaps, what they need isn't a door they can open and drown themselves with. Seems like what they need is a more effective location broadcast.

We don't actually know that they don't have good ways of being found. If they do, it makes it all the more likely that the sub is on the bottom :-(