Had the same problem on mine and it turned out to be a bad connection of the alternator earth. If you have a volt meter you can check across the battery to make sure its charging
Im not sure on the swap parts between the corrado but pretty sure there is a list on the net somewhere. Im pretty sure you can swap over the complete front subframes with the mk3 but you will have a slightly wider track so may need to roll your arches out a bit. Also with my car im pretty sure im running corrado shocks to better compensate with the extra weight over the front axel which the mk2 dampers wont be set for. Good luck with the conversion
When running your voltage should be over 12v across the battery. Not sure if that could relate to a bad earth somewhere. There will be a transistor switch or relay between the ignition switch and the circuit for the starter solenoid
For coolant change on my car i undo the lower radiator hose and let it all drain out that way into a bucket. Then just put the hose back on and refill it. Not sure about gearbox
How much you want for them tony send me a pm , Yea i know throttle control but thats the thing you have to hold back while the others just put there power down. Looks like paul has the right idea!