Did you already finish the nice cathedral you were working on?

walls cannot be higher than 2.000 cms (two thousand)

In fact there's no real limitations in theory. It's just because the spinner controls with -/+ that let you choose lengths in Sweet Home 3D need a maximum value. I felt a 20m high wall would be ok.
If some users like you or other ones need bigger maximum size for any field in the user interface, please feel free to share your ideas here or better in a request. I'll change all of them in once.
Meanwhile, here's a workaround : use meter unit in preferences because the 2000 max value don't depend on the chosen unit (but that will probably change in the future).

there's a twilight zone where illumination is switched on and off

This a bug in Sweet Home 3D lights management and I just fix it. I never thought Sweet Home 3D could be use to draw such big buildings! [;)]
Expect a version 2.3 Beta in the coming hours to let you work normally on your Pantheon.