Personally I think a diplomacy win is a non-starter.
If you stay Buddhist, you'll never, ever get Isabella's vote.
If you stop being Buddhist, you'll lose Ragnar's vote (you've just lost the favourite civic bonus by switching out of HR and you'll probably get another -1 when you redeclare on Toku).
Yes, it occurred to me that Ragnar may be on the borderline of dropping from Friendly to Pleased, and declaring war on Toku again might push him over that line.
I was looking at the tech tree again and at the happiness rates in my cities just before I made peace with Tokugawa. To finish him off, I will likely be gearing up to handle war weariness: building markets, mausoleums, theatres, and temples, and researching Industrialism (cheap factories with Organized!) and Fascism. Then building the Pentagon and Mt. Rushmore.
Frankly, that sounds a lot like a build-up towards a domination win. If I went for diplomacy or space race, I'd be switching my focus on a dime once Tokugawa was either dead or my vassal. Ragnar might have a head start towards space race at that time, and I'd be playing catch-up. He'd also probably get stingy with his techs as he closes in on a space race win.
So maybe I should suck it up and go after Ragnar after all.

If I can get a tech lead on him thanks to the GA infrastructure builds and switching to Free Religion, I might beat him to tanks. Oooo, tanks. Fun. I'll also have a big army of veterans once my war with Toku is over. I can empty my own and former Japanese territory of most of my forces and distribute them along the border cities. If I can get railroads set up by then as well, I should have the strength and the mobility to deal with anything Ragnar throws at me.
The nice thing about the other continent being behind in tech is I doubt they'll interfere on my continent. Churchill will also be quite content to pursue a space race win.
On the civics changes--no, I wouldn't make this many with a non-spiritual leader, of course not! But it makes sense, as others have said, to take full advantage of the trait. And it's fun!