1. to connect, as in the picture, put the CT on three wires (A+B+C), or on four wires (A+B+C+N)???

2. do not need to connect CT2?

Hi:

1 Ct only need to be installed in live line.
2 CT2 is not necessary.

In fact, the meter type of "load"(as the ct2 in above pic) is not necessary in the solar pv monitoring application.
You can use it to monitor any load circuit you want to monitor, but it will not affect the overview display or report calculation in solar pv monitoring application.
You can just take it as an independant circuit loop monitoring.