Also, in FS when signal loss is activated, my throttle goes to about 54%.
When FS is activated with switch B the throttle stays at zero. Is this correct?
I think that's not ok. If true FS is activated (e.g. by switching off the transmitter), throttle of 54% should be fine, I assume.
But throttle <10% while in "fake" FS flight-mode seems to be a bad idea, if I read the WKM manual carefully. I would try to fix that by a flight-mode specific setting on throttle or a flight-mode specific mixer on throttle or anything like that.
You can easily double-check with the WKM assistant software, there you see the throttle value as it is seen by the copter.