First scenario - if the intruder is clearly unarmed and poses no immediate threat to life or limb, then, in my state, I feels free to draw and hold him at gunpoint if needed until police arrive. If he turns and runs, I let him leave. After seeing then business end of my weapon, I doubt he's ever coming back to my house. Call the cops anyway, provide a description, and file a report. If he's armed, then he gets fired upon before he gets a chance to fire at me.
In the other two scenarios, I'm finding cover with gun ready to be drawn if needed, calling the cops, and being a good witness. There are just too many repercussions for jumping in to help a stanger that isn't obviously about to be seriously injured or killed.