Re: Engine surging
Might be the idle speed control valve. I think the circuit only works when it sees little (or no) road speed, so I wouldn't worry too much about the idle speed being worng whilst coasting (although 2500 sounds a bit fast)! As it comes to a stop, the system will kick in, realise the idle speed is way too high, and drop it. There seems to be a bit of "overshoot" in your case, where it goes too low and then messes about a bit until it gets back where it was. I believe there is a "system reset" you can do (hopefully someone will be along who knows!) but it's something to do with turning the ignition on and off for a particular period of time.
You could also try putting the car in 5th with the handbrake hard on and letting the clutch out SLOWLY to try and slow the engine down at idle. The idle speed control valve ought to "fight" you and maintain the idle speed (as far as it can)! If you then suddenly depress the clutch again to remove the drag, the idle speed should shoot up (too high), momentarily and then drop back down to its correct speed.