The one thing I would change is the way leadership works. Instead of lowest to highest why not make it the other way around (highest to lowest). The you can use the sum as a sort of currency to do actions. ie. action points. This allows more flexibility than just spending die.
For instance, you roll the 3D6 and get the sum. That represents how many action points (AP) you have that turn. Certain actions then cost AP to perform. For instance, scouting a tile might cost 4AP. The player then removes that from his sum represented by the dice in front of him.
This allows for more actions per turn. Cards to modify this sum (negatively or positively) to provide some added strategy and to supplement the diplomacy.
Personally, I would not want to resolve diplomacy directly with the roll of a die. Free-form diplomacy always works the best (and is the most fun). You could allow people a number of diplomacy options then set the number of AP it costs to propose the action and offer player a chance to "sweeten" the deal. The player being propositioned can then propose a counter-offer which is then accepted or not.
For example, if I roll a 2, 4, 6 on my turn I would have 12 AP to spend. Imagine I really want a 2 season truce with my neighbour (Tommy). So I consult the chart and see that it costs 4 AP to make the proposal. So I remove this from my dice total immediately (2,6 = 8) that is my risk since I dont know if Tommy will agree. I look through the chart to see what I can offer, ie. troops, AP, die, territory, etc.
So I make an offer for 6 AP this turn for 2 season truce. He makes a counter offer, which cannot change the term (2 season truce) but can change the incentives. So Tommy offers the 2 turn truce for 6 AP now and 1 die next turn. If I refuse the initial AP is lost. If I accept the agreement goes into effect immediately.
Diplomacy would have to occur earlier in the action list (likely first).
Hope I explained that properly :S Once you get the game finished let me know I can help produce it for you :)
Field Marshal Games