I appreciate the sentiment. I don't think there's necessarily a "right" way. It's a balancing act. First of all, information evolved over time. Secondly, I have to balance what I think is right along with my wife (who I have to also make sure is happy in this) along with what we can likely accomplish with the neighbor with a certain amount of effort/force. I was glad to hear my wife wanting to take emotion out of it last night after reading his email and being content with his proposal. Because ultimately that's more important to me than making neighbor guy "pay his share". Inversely, this would have been more difficult if she was pushing for more knowing that my options would be limited. So while others may have handled it differently, the best course of action is what works best for everyone involved. I feel good about that for our situation.
I've called the nurseries that have larger trees available. The cost of a 12' Green Giant Arbovitae is about $1300 planted. This is a good alternative as it will provide privacy and grows quickly. The problem is, looking at the area where we are to put these, 2 of these may not be enough. 3 would be ideal. So, with this information, I may be going back to neighbor and explaining that we are interested in getting this done without lawyers, we've looked at alternative trees, here's the cost, and it's going to be $4K to fix it.
I don't know what his response will be, but I think it's a reasonable counter to his email and close enough that I can't imagine we still can't get this worked out. And, in the end, if we can only plant 2, it'll still be a good solution.