creampuff. That's pretty technical for a bush lawyer!!
The voltage in the secondary coil is generated by the collapse in the magnetic field of the primary coil and is dependant on the rate of collapse of the magnetic field. Its governed by Faraday's law of Induction. I guess you could say that the magnetic field has collapsed with the current is zero, but its about the magnetic field, not the current.
I used 10 amps in the primary only to make an estimate of the current involved in the spark. But I think I got it wrong. I think energy should be the common parameter not power as I did. It did take me 2 goes to pass my electrical engineering subjects!
The point I was trying to get to was that the amount the spark voltage is diminished by the spark plug resistor is dependant on the current. And the current is bugger all, so the voltage drop from the spark plug resistor is small.
My good friend Google tells me you need a minimum of 12,000 - 25,000 volts for a spark which is consistent with other figures. I think your 2,500 volts might have a decimal point wrong. The Bosch handbook will be the oracle, but its at work. I'll check later.
The point I was trying to make was that I don't think the resister per se is the issue. I think the changes in resistance you measure are indicators of insulation breakdown, not deterioration of the resistor itself.