1. You just go to super cruise as well, it is as simple as that.

2. Not necessarily. If you are behind him and see where he goes, you may stop (or throttle down to 30km/sec) and go through the celestial bodies in the left panel, target them. you may be lucky to find the one in the direction he was pointing at, so you know where he jumped. This is possible - but time consuming and annoying - and if he jumps again after reaching his destination, you wont witness that. Here a wake scanner is a life saver, obviously.
And yes, you can scan in super cruise.
Note that most people will go to super cruise to fly to a near celestial body of that same system. No one goes into super cruise to then jump to another system... well a noob who doesn't know better might do that, but it is pointless cause you can jump to another system straight away.
Yes, if he drops out from super cruise, there will be a white target on the radar. Depending on what he did (dropping out or jumping away), you'll find the according wake, even without a scanner.
Just keep looking, they are there.
Edit: Note that NPCs, at the moment, unfortunately ALWAYS jump to another system when they try to get away from you. This must be a bug or unfinished feature yet, cause it makes not much sense. They jump quicker than we can (assuming they go to super cruise) but leave high energy wakes... so to find these NPC's you will need a scanner for now I'm afraid.