"Zubrin is talking pressure, not buoyancy."
No, he describes how to make this huge anchor:
"...assuming typical martian dirt with a density four times that of water, then the skirt would have to be anchored about 10 meters deep in order to have enough mass weighing down..."
Etc

"A properly-designed pressurized tensile structure over, say, a crater, could create a very large space. It would be under a lot of tension, and I don't know if you could do this with existing materials. I haven't researched it."

Well the tensile strength would depend upon the air pressure inside the crater- no pressure which above Mars' pressure in crater could result in needing a higher tensile strength than compare to enough pressure to lift enough to cause it to sag less from the weight of the material.
So one could simply design it not to depend upon to relief from the pressure [and so if you lose pressure, that doesn't cause it to collapse]. Now, if you want 1 atm it's going to require a lot of structure- and could be rather complicated- on scale of bridge building.
A 1/10 of atm or 10 times Mars pressure shouldn't be too difficult.
As just wild guess, if you wanted 1/4 or 1/3 of Earth atm, it probably would require cables about 2" in diameter.
And anchoring- though probably not as much as Zurbin wants for his 50 meter dome- for crater around 1 km in diameter [possibly larger diameter crater].

"A membrane with 14 psi internal pressure would be "balanced" if you put a bit less than 14 psi of weight on top of it. Maybe a double membrane with water between them? A square foot of membrane has about a ton of atmospheric pressure pushing upward (if it's a near vacuum outside). On Mars, about ninety feet of water would balance this (very rough numbers). I'd use less so you had a good upward bulge in the double membrane. Water would stop a lot of radiation."

I don't think you want to go for 14 psi.

Maybe at most you could go for Denver pressure [12 psi]. But going Peruvian better still.
And frozen water ice could add some to the structural strength- not having it ice, will cause the need of heating it- unless you need dump heat through the ceiling.
Or you need to keep entire space fairly warm for some reason. You could just want space in which you can walk around and work without a pressure suit, but if you want the whole area as greenhouse- significantly above 0 C, it might make having an ice roof a problem.

"I'd have to sit down and work out other issues: the water freezing into an ice dome and ceasing to load the membrane evenly is one issue."

The expansion from forming ice is rather complicating factor- possibly very explosive. And trying to make the ice extremely transparent won't be easy- and having a CO2 atmosphere won't make it easier. Take straw blow into water in a glass.
Might make ice bricks and weld them together.

Or perhaps something resembling a ice cube tray.