1. Do what the situation calls for, if you can move over right then do so. If not, just try to get by quickly and safely, should the HGV start changing lanes give the horn a go, if that does nothing drop back sharpish.
2. Officially? To alert others that you're there. Unofficially? It's rather acceptable to give someone a blast when they've done wrong. A quick pip when someone isn't paying attention at a set of lights is also somewhat acceptable.
3. The other drive could've likely seen the cyclist. Don't let people pressure you into doing something that isn't safe.
4- Two things here really. Firstly, if you believe high RPMs for the car are bad, you need to drop that belief. Providing the engine and oil are up to operating temp, there's virtually no wear and tear on the engine, be it cruising at a leisurely 1500 RPMs or blasting away at 6000 RPMs. Yes, it's noisy, the car sounds like it doesn't like it (older diesels especially) but it's just the nature of the job they do, unless they have a pleasant engine/exhaust note, they're going to sound like they're about to self destruct (again, especially older diesels). The more power you need to tackle a road, the more RPMs you need. To add to that, when you shift gear your RPMs drop anyway but then you've also go gravity slowing you down while you're freewheeling. You need to shift at a point that puts you in the effective power band for that hill when you get into the next gear. For my car (1.4L turbodiesel putting out a huge ~69 BHP) you're talking about 2250 RPMs after you've shifted gear (roughly where the boost from the turbo really starts to kick in), so that's shifting at about 3000-3500 RPMs.
Don't take that to mean that's what your car needs. Each car is different. A lot of modern cars have the turbo kick in at lower RPMs for efficiencys sake. Likewise, more powerful cars produce more power at lower RPMs. While my car is producing ~150 NM of torque at ~2300 RPM a Ford Mustang will be producing that same amount of torque below 2000 RPM. So for that Mustang, it's effective power band for a hill is a lot lower than that of my car, also helped by the nature of the engine itself. High displacement petrol engines with a lot of cylinders are very diesel like in the sense that they produce a lot of torque at the low end (aided by the extra cylinders and longer strokes), not quite peak torque at very low RPMs, but you're talking 400NM of torque (out of ~475 that the engine will produce at peak) at about 2500 RPMs.
Secondly, on steep hills, timing is everything. You can't be a slouch with the gears on steep hills else you're going to need to compensate with more RPMs.