It occurs to me that what they could do with is a Mr-like "escape capsule". A diving bell, moored to the sea floor, with slight positive buoyancy, on a mechanic

It occurs to me that what they could do with is a Mr-like "escape capsule". A diving bell, moored to the sea floor, with slight positive buoyancy, on a mechanical tether would allow the aquanauts to ascend slowly if there was any failure in the habitat. I've not seen any information that such a thing exists, but a low-tech device like this would seem to be a prudent idea. On the other hand the habitat is at 19m, so even a small amount of steel walls are unlikely to suffer an "unexpected breech."