those 3 levels you speak of are ways to change our consumer patterns by planning our infrastructure more wisely.
however they are unrealistic  in our world, as to build these we'd have to destroy a lot of infrastructure first. And, who is going to pay for this new infrastructure?

Hence the idea that with our CURRENT levels of consumption, our world wont be able to support our population.

And it is irrealistic to believe that our levels of consumption will change drastically for the better, hence to balance things out it'd be easier to change our population size by reversing population growth from positive to negative leves. THAT is the basis of Malthusianism.

lmao! nice burn zarfy

It is not realistic to use the earth's entire volume in these calculations. There's only a certain depth in the ground that humans can reach to inhabit or use for transport etc. Any deeper and you reach molten rock and metals and your body starts to melt, not to mention the extreme pressure and lack of oxygen you have to face.
Next there's the problem of reaching deep depths in water. Here the water pressure will simply make your construction implode.

Aside from that, Wornstrum's and your, xeno, numbers included the atmosphere. I'm pretty sure we will not be able to effectively and efficiently make use of any layer in the atmosphere above the troposphere, not for production of energy using solar panels (subsequent layers all the way down to earth would interfere with each other's efficience), let alone for residence (oxygen levels too low, UV levels too high, etc.)

Math or no math, overpopulation is a relevant issue.

Easy solution : Introduce cannibalism to overpopulated areas ; Solve overpopulation and hunger in the same strike. Also, lock the borders of the overpopulated areas. Problem fixes itself. Unhuman? Are their current living conditions human ?

But looking away from my easy fairytale solution, how to deal with overpopulation will never be increased government control. Which would be needed to arrange a smart three layer structuring to how we organizeour world. Fact 1. We're all different quirky individuals. Fact 2. Some of us like to live on this planet. By these two alone one will find it difficult to convince -everyone- to move into same area(layer) in equally spaced living-zones. This by fact 3. We will always consider some people to have less value than ourselves.

Call me an egotistical maniac sure, but you will pay 1 milllion dollars to the guys who kidnaps your sister. You will not pay that amount for the release of a random kiddierapist. Just saying.

So how then to combat overpopulation sensibly ? We don't really have that many options.

1. Decrease population

2. Increase area that can sustain life

Both have ups and downs, and certainly both will have humanitarians tearing their hair off in frustration over risks taken and borders crossed, but is it right for humanity to perish because we're to humanitarian to save humanity ?

I'll leave you with that non-sesical question for now.

"we'll not reach 20 billion in my life and likely never will"

We could reach 10 billion by 2040 if we keep on our current neo-malthusian projection where pop growth is dramatically limited by artificial induced scarcity of opportunities for the fulfillment of basic human needs:

http://en.wikipedia.org/wiki/File:World-Population-1800-2100.png

But what if the problem of scarcity of the opportunity for the fulfillment of basic human needs were suddenly solved?  Additive manufacturing processes and A.I. have the potential to do this today.  We made fiscal injections into a banking system which will waste the money on inefficiencies inherent in their antiquated processes, their own bonuses, etc., essentially propping up a system that should've been left to fail.  We might otherwise have been able to solve the problem of scarcity of opportunity for the fulfillment of basic human needs by instead injecting the same amount of capital into automation of distribution infrastructure, resource extraction, and additive manufacturing processes.

The photo I linked to in previous post in this thread as well as these concept drawings express somewhat the vision I have for how humans could live sustainably:

http://3.bp.blogspot.com/-zAIP3HGWUFE/T0BuR1xlNkI/AAAAAAAABqQ/YqIm0FiJjsQ/s1600/Vertical+Cities.jpg

http://ynotwhy.com/wp/wp-content/uploads/2011/12/sgdk6z8s91g5hppn.jpg

http://www.livegreenblog.com/media/photos/38/6604/Flat_Tower_1_lvgmain.jpg

My vision is a combination of the 3 concept drawings above.  Pre-fabricated living units produced with automated additive manufacturing processes (technology that we have today), would be attached to a honey-comb lattice-work dome-apparatus, which is essentially one huge solar array constructed above agricultural land.  Each section of the lattice to which the living units would be attached would themselves each produce food, too.  Each section of the lattice would have to give leeway to and yet also support each other adjacent section.  This would allow the entire dome to mitigate effects of any tectonic shifts.  In this way, there's virtually no limit to the size of the dome that could be attached to the Earth.  Each section of the lattice (just like each living unit) could be detached and replaced as necessary.  Sections would be aerodynamic in form, so as to reduce effects of wind, produce lift so as to reduce load on adjacent sections, and so as to more easily transported by air.

The scale of these domes could be enormous, with diameters of hundreds of kilometers, each section designed with ultra-light-weight materials and design (current automated additive manufacturing technology can accomplish this). The living units and the sections of the domes' latices, would be mirrored on ground facing surfaces and would be solar arrays on sky-facing surfaces.  The mirrored surfaces would produce an albedo effect to redirect sunlight back towards the surface, creating a greenhouse effect on the land beneath.  This would create a low-pressure zone within the dome where heat updrafts would help to further diminish load.  As such, these "living arrays" could more or less float above the landscape without dispensing much load at all to the surface.

In no way could mid-/low class housing support any such structures being built on top of it, lol.