InputActionEvent name is taken from Config ini files, when you have input defines in your Project Settings’ input array.
2: No idea without reading code; there’s zero explanation from Epic on how K2 Context Compiler work.
3: You can instead use a UFunction with Params that are set to default values; than mark that UFunction as Internal Use Only, then make your custom K2 node capture such default values when node instance is created. I did it this way for my Object-Pool plugin’s custom Spawn Actor nodes.
4: Old Kismet stuff; again, no docs for this.