1. That tapping will be your hydraulic valve lifters........very common problem. First of all, have you got a genuine VW oil filter fitted? It has a non return valve in it that prevents the oil in the top of the engine returning to the sump. The cheaper filters don't have the NRV, therefore the top of the engine is starved of oil on startup.
If its not that, then you need to use some lifter cleaner to give them a good clean out. 'Justins145bhpMK1' should be able to help you out with this one.........he works for Castrol and seems to know his stuff.
2. That strange noise under acceleration...........does it sound like a tinkling noise that eases if you ease back on the accelerator. If so that'll be pinking/knocking. You could be right it could be the ignition timing needs retarding. However what fuel are you using? If you're using 95RON unleaded, try some SuperUnleaded (97RON) or Shell Optimax (99RON).
3. Sounds like something is loose, you're going to have to get under there and have a look around. Sometimes well worth doing this with the weight of the car on the wheels i.e on ramps, as when you jack the car up the suspension and subframes drop down and the thing thats rattling i.e exhaust, may have larger clearances.
4. You're sensor isn't located correctly and is getting a misreading from airflow passing over it. Not sure where yours should be, but on my Mk1 its located behind the bumper. Yours may be located in the inner wing somewhere?
Hope that helps.........good luck
