I had the exact same problem. Tried many saved games and tactics, but always with the same result... Galmar and Ulfric wouldn't engage Rikke and Tullius. After several tries, I noticed I never actually met Ulfric outside Solitude and heard his speech. I'd seen it mentioned in Forums, but didn't put two and two together. So I reloaded again and met with Galmar at the Haafinger Stormclock Camp, but this time I fast travelled to the Thalmor Embassy and traversed over the mountain down onto the gates of Solitude, heard the speech and ended the scenario properly. Don't know if that helps anyone, but it worked for me.
