RalphChristie,
Lightning can be a tough problem. Don't forget that the impedance seen by the lightning is not the impedance seen by the normal power system due to the frequencies involved. In response to a couple of your points:
1. This will occur only if the strike was close to the substation, or if your downwires (connections of your OH earthwire to your earth grid at each pole) are not in good condition. We have had part of our system where the number of lightning strikes and the corrosion from the soil have corroded nuts that connect the downwire to grid. This essentially raises the potential of the downwire to a high voltage, and can flashover thhe insulators on the phases as it passes by them. Also, you might want to check that your earth grid at each pole is of a suitable resistance. Again a high resistance can produce higher voltages where you don't want them.
2. Don't forget that the clearances must be good enough for the expected lightning potential that may occur. Again if the earthing system is not performing properly (either from degradation or poor initial planning) these clearances may not be large enough.
3. Good. Are they working properly?
4. Earth wires are run over the top of transmission lines due to a number of factors. You may also have noticed some of the lines have an earth wire that runs for about a mile (or similar rough lengths) out from each substation, because if the lightning strikes occur further out than this, the impedance of the line is great enough to attenuate the lightning strike to a level where a reasonable SA will protect the substation. You may decide that the earthwire is not required, but don't just remove it because you think it is causing problems, if you do remove it, ensure you look at the total insulation design of the susbtation and line, like incresing the BIL of equipment in the sub if requried.
5. You will have touch potentials at a wooden pole, they just may not be as great as a steel tower or concrete pole. You are dealing with large currents (short term) and these can produce large step and touch voltages, especially if your pole earth grids are not working as deisgned.
Please be careful when dealing with protective earthing, and do not take a decision about changing the condition of the system lightly. Questions I would be asking are:
Was the system designed with a certain lightning strike input, or was it designed off the shelf?
Has the performance of the system degraded (more trips) recently, or has the problem been there since as long sa everyone's memory serves? If it is more noticeable recently, then I would say look hard at your earthing system at each pole. Simple earth grid resistance checks can tell you a lot. Maybe do high current checks across the connections at the top and the bottom of the pole to look for problems.
Get a good insulation co-ordination engineer to go through insulation levels before making any removal decisions.
As with a lot of power system engineering problems, the symptoms don't always point directly towards the problem.