So gave this a shot....haven't gotten back to it in a few days.....am around turn 550 I think....am only 50-60% on two remaining Civs. Used the advice and went with Isabella, Genghis, and Swedes. Was an odd game start as Sweden almost acted like they were playing OCC. They were on an island with me and never even saw them make a settler. Then them and Mongols declared on me, so I took out the Swedes. Isabella is over 60% but Mongols only around 50....getting 34% penalty for different ideologies. Even though he has 2 cities. Not sure why Isabella didn't wipe him out completely.
The good thing is I am faring quite well for my first Emporer game. Got religion...got Sacred Sites. Took first prize for Worlds fair and Intern Games. Built some wonders...though it's tough. Got Louvre and a few others. But Isabell built Apollo Program so can see writing on the wall. I think I can win Diplo victory, maybe before she launches. She has been quite far ahead in tech all game.
At this point, having never even played Emporer, I think I'll take any win, is what I'm saying. For the Achievement if nothing else.

I'll forge on for a bit but the culture war is rising far too slowly. I've got close to 200 tourism too!
Edit: There you go....turn 642 I won a Diplo victory. Ended up unlocking 3 achievements!

So couldn't pull off this scenario, but my first ever Emperor win and I'll take it.