"he sighs and tells me I focus too much on minor, irrelevant details"
The direct response to this is to point out that he was responsible for a large delay in executing the idea and it is not a minor mistake if it takes him that long to understand its impact. Pointing out the impact on the company, an irrefutable common value, is much harder to dismiss.
Just my personal experience, dealing with narcisissts in general require the cold and discipined approach. If they refuse to reciprocate social graces then the only option is to argue from common reality.
Not sure what to suggest about your upcoming lunch meeting but if my advice makes any sense it might be better for you to take a diplomatic stance and continue tough it out against your cofounders.