So if the power goes to earth, in my view, it makes no difference whether the plug gap is closed or the plug is connected to the ht lead and earthed away from any ignitable fumes or the ht lead connected to earth..
Actually, there is a difference, although any ignition system will be designed to easily survive the difference. Whether the plug gap is closed and it is earthed, or the HT lead is connected directly to earth, it is the same. However, this is different than if the HT lead is connected to a functioning plug that is earthed.
In the former case the HT lead is shorted directly to earth so the voltage inside the coil never rises above ~0 Volts. In the latter case it rises to ~1 kV (for a gap of 0.018" at room pressure) before a spark occurs and it pulls the voltage inside the coil back down to ~0 Volts.
Again, any functional ignition system will easily survive this difference, but there is a difference. Also, I wrote ~0 Volts because current is flowing so there is a voltage drop across the coil, but it is quite small compared to the 20 kV overvoltage situation we're worried about if a HT lead is left dangling in air.