Alright, fine, I'll finish this thread off myself.

I chalked the long campaign that never ended up as a victory and proudly moved on and started up another one, short campaign this time, to see if I'd get the same results, hope, as they say, springs eternal. And despite battling through some desyncs we persevered.

I took up command of the Mori, quickly established all the trade nodes and control of the sea. My ally took up the land campaign as Date marching towards Kyoto from the opposite side of the island.

Early on the Mori had it all their way easily taking all the provinces leading up to Kyoto, establishing a sound economic development, ample income and then settled in to wait for the Date to catch up.

The Date have a long march, so by the time Mori had 8-10 provinces under their control, Date had 4, and while they could field a decently developed army early, they had to march half way across creation to get there and didn't make a very solid income (fortunately their wealthy Mori ally had deep pockets and sent a constant stream their way).

By 1555 the Mori have Kyoto surrounded, Date have pushed up into the middle plains, combined the two controlled 31 of the 35 provinces, excluding Kyoto.

Final push is applied, Kyoto is taken by the Mori before realm divide, the Date swoop in on Hojo and Uesugi and destroy both pushing the province count to 39. And in 1559 Kyoto being held for a year, the victory was claimed and the achievement awarded.