Hello and welcome. Please let me answer your questions:
1) This is not implemented, but I can add this 🙂
2) Some few actions/conditions (called Tasks in NodeCanvas) do have similar debug gizmos. If you are making custom action/condition tasks you can override the OnDrawGizmosSelected method to draw gizmos since there is such a call for this purpose. I will review the existing tasks in case there are some that could use gizmos but are currently not.
4) Live variable values can be displayed but at a different place, that being next to the variable in the Blackboard the variable is (The Blackboard is a place where Variables live). Since the Blackboard is always shown in the graph editor, it also makes the variable value visible at any time as long as the debug live value is enabled for the variable. It is not by default but it can be enabled selectively for any variable you want.
If you have any other questions, please let me know.