The FSE Override is only meant to be used in cases where you can't use the custom Load Manager (such as using FS Economy where it sets the payload). Some features are not available when in that mode.
Regarding 1. We have our own O2 and TKS system that is active whether in FSE Override or not. We don't use the default system for that, thus the sliders on the default W/B screen mean nothing.
Regarding 2. X-Plane handles those units there, we don't.
Regarding 3. Again, that would be an X-Plane issue where it isn't saving the weights stored there.
Why they work on the default SR22, I'm not sure, but again, FSE Override is not intended to be fully featured, as we have heaps of custom systems that are best interfaced through the custom load manager.