OK cairns. a couple of observations as I said I would.
Your top sentence in yellow, I take this to mean that a player can bombard their opponents targets. But why not allow them to bombard their own ones as well along their line. This would allow players another strategy. Hit an opponents to stop them or their own to lower their own neutral count when moving of from their base.
When you get to the centre, you have four spikes, 2 gold and one silver. One that is silver and gold. This gives the top player an advantage as when they get their, they have half of the spikes needed. I understand the need to make both sides slightly different but would you put different neutrals onto these two to make it better for the player coming from the bottom?
Or the second golden spike, how about a name change to ceremonial spike (the one kept by the family). Place this at the top and only have one gold and silver spike to fight over. this negates the balance issue completely and allows the spikes to have completely different names overall.
You have native Americans attacking the line, would you consider adding maybe an army base to both sides with an auto deploy? This would allow more troops to defend the lines bombard points. Without it, I fear that no one will ever keep a bonus.
Been looking around to see if any part of the line was started and then abandoned because of bad ground. Did you find anything like this in your research? Might be nice to say have a false start along both lines. This can then be a conditional border. You have to grab the false start before being able to continue.
Lastly, just for the hell of it, add a losing condition of holding your starting point or you lose the race. Should be fun with nukes, not that they had them then.