Do you have a multimeter? If so, take the boss off and measure continuity (resistance) between the ring at the back of the boss and its chassis. If you have 0 or very low ohms, your issue lies in the boss and not the method you're using it to fix to the car. It's entirely possible something has moved in the boss and is constantly earthing out the horn push wire.
Assuming you've kept on top of your maintenance. Give it a major service before you leave and swap out things like brake pads, check your brake hoses for degradation, replace the flexible fuel hoses going from your chassis leg to your fuel rail/metering head, check all tyres, wheel bearings, ball joints and bushes. Go over the car and fix any coolant/oil leaks you may have, swap any dodgy looking coolant hoses, you can get them all brand new off autodoc now for only a few pounds, so it's not even expensive to do. Really give it a birthday. Do it a good hundred miles or so before the trip, so if you upset anything you'll know well in advance. Doing this will dramatically reduce the kit you need to take with you and prevent any unwanted stoppages. Then, I'd keep things like: Coil pack Spark plug leads Dizzy Cap Rotor Arm
And a basic tool kit.
Been going to the Nurburgring for about 20 years now and this is what I usually do.
Given the location it could be something else and the wet patch on the door seal could be red herring. Could be the fresh air intake in the scuttle.
My thoughts too, while you're checking the scuttle, make sure the drain holes either end aren't blocked. If they are, water will pool here and find a way into the car.
Yeah, those bypass valves will be the problem. They were only installed as a recall because the original mk2 matrix couldn't cope with high pressure and would fail. So they were designed to bypass in the event of an over pressurisation. Though nowadays the fail and bypass for normal operation. If you've had a new matrix, it will be the newer design (rolled out in 1992) and you'll be safe to either bypass with a piece of metal pipe or if you're feeling like it, you can buy the rubber pipes that run straight from the matrix to the head/crossover pipe and do away with the valves that way.
Take a brave pill and if possible, use one of those plastic trim removal tools. Worst case, you break some of the little white clips that hold it down and you can buy more of them.