Thanks nallath,
It has total sense reducing the actual Z volume to "gantry height" as that is the limit. Silly me not having realized.
There's still some issue regarding the boundaries for the nozzle when "one at a time" is enabled even for a single item in bed. As you can see in the picture, 4.3 creates that dark gray area (that seems to be unrelated to printhead dimensions, as modifying those doesn't seem to change it).
This is a comparison between 15.04 and 4.3. Both without any adhesion feature but a tiny 2 lines skirt.
It migtht be related to Creality printers profiles. Moving to my standard Anet A8 profile the square light gray area still appears, but it allows you to go to the bed limit (as long as the dark-gray skirt is inside).
Neither my Ender2, nor a default CR-10 profile allows me to go to the limits, as the dark grayed area is a square almost the size of the light gray one.
I have discovered that in the Printer Setup, "Extruder 1" tab, neither of the Creality printers have a Nozzle Size field, as my Anet profile has. This seems strange.
Adding a new Ender 2 printer with a clean profile shows the same "missing" fiel.
I guess I am not awaare of some configuration with this new version. Sorry to bother you.