It's worth contrasting the design of this vehicle with what is ostensibly the world standard for deep-sea research submersibles, Alvin of Woods Hole Oceanographic Institution.
As a US Navy-owned submersible, Alvin also undergoes extensive safety evaluation under the Naval Sea Systems Command SUBSAFE program: https://ndsf.whoi.edu/its-official/
I think it's 30 days of diving, not 30 continuous days submerged. You can use it a whole month before you need to take a maintenance day.
There's a 3d panorama view of the interior of the sphere linked from the article - I doubt anyone's spending more than a day in there at a go, let alone with someone else.
Edit: apparently, it's 3 people - the pilot and 2 scientists.
Other manned submersibles undergo classification from groups such as DNV: https://www.dnv.com/services/manned-submersibles-1102
Yet OceanGate seems to have avoided this kind of safety classification: https://oceangate.com/news-and-media/blog/2019-0221-why-tita...