The Billy shelves do come in half height - see item 202.638.30.
For the corner unit you are buying 3 shelving units say 202.638.30 and one set of corner fittings 401.041.09.
Your problem with the low shelves is that the top will be open. It is also open with the taller units you just can't see that it is.
Bring up the corner fittings (you might need to hover over the pic of the fittings and then click on the shelf unit that appears, scroll down, click the product details and then the assembly and documents and open up the instructions, you'll then see how the corner unit works.
You'll also realise that you only need to use the 40cm wide for the unit in the corner, the others can be 40 or 80 wide.
