When the engine is started from cold, the ECU makes the injectors squirt more fuel, to make the mixture richer.. more likely to catch light. It's like the old days of using the choke on a carburettor'd engine. Injection systems replicate that electronically.
Once the engine starts, that rich mixture is tooooo rich and the revs rise, so the idle control valve is there to tell the ECU, so that the ECU can lean off the mixture to keep the revs at the right level (850 rpm +/- 50).
I have started and moved a car just a few feet... and that seems to confuse the ECU so that next time it tries to start it doesn't use Cold start mode.. it turns over on a lean mixture and doesn't fire.. so the engine floods.
A flooded engine (unless you drove through a creek) is when there's too much liquid petrol in the cylinders. Fuel has to be a vapour but if the beast isn't catching, then the fuel condenses out inside the cylinder and makes the spark plugs wet.. Wet petrol doesn't burn all that well... it needs to be hot and vapourise..
The normal cure for a flooded engine is to take the plugs out and dry them off. If you can heat them up too then that helps the petrol to vaporise and you're back in business.
It's easier to start a flooded engine using the technique I described above. If you hold the gas pedal fully open while you turn the engine over that lets a lot more air in. Normally this prevents a cold engine from starting as it makes the mixture too lean.. but you want to flush all the unburnt petrol out so it's okay.
"Hot start" is or was advice when the engine is hot, just to hold the gas pedal open a tiny fraction. It prevents an engine from flooding, if it's warm enough to start without being slightly rich... though modern ECUs and injectors are a bit more clever than they used to be and this technique might be old fashioned. Anyway I think your engine is probably flooded rather than hot.
I'd leave the starter to rest for about the same amount of time that you cranked it. So crank for 5 seconds.. leave it 5 seconds.
The backfire sounds interesting... you must have fuel and something is trying to set fire to it..
You don't want tooo much of that since it can blow your exhaust baffles out (if the exhaust is a bit frail...
) or damage the catalyst. In some cases you might damage the manifold (yours is metal, so no worries)... but don't be too alarmed by it. Just a lot of unburnt petrol... which at least proves the fuel is getting through. I'd concentrate on the sparks. Are the plugs firing?
The buzzing is the fuel pump and injector relays... the pump is in the boot. But anyway, the fuel rail test will prove if the pump is okay or not.
The fuel rail is the square section tube behind the inlet manifold, as you look into the bonnet. It has two rubber tubes on the left hand side. I think the one furthest left is the inlet (from the tank) whereas the other is the return. You might be able to feel the fuel pipe "pulsing" which would save you disconnecting it.. but your back-fire probably proves it's working.
I'd check the sparks now and if that looks good, just concentrate on the "hot start".