If Fredrick Augustus fails to secure the polish throne, it means that the Prince de Conti becomes the king of Poland. This is problematic, since Russia and Austria were opposed to a french-allied Poland. You could very well see a war of the polish succession break out much earlier.
Glad to help;
Now, you said you wanted north africa in this Gothic empire. I would suggest that Hilderc put up a better fight against Gelimar, leading to a big Nicene/Arain Vandal civil war. This "Gothic emperor" could offer to aid Hilderic in exchange for him becoming a vassal king.
If that...
Your going to need a Pod that is early enough to keep Aquitaine under Gothic control. With Aquitaine under control, the goths can dominate most of Gaul, which would provide the necessary man power and tax base for a "Gothic empire"
For political stability, keep the Balti dynasty from...
According to the encyclopedia of Islam, Muhammad sent a letter to Khosrau II demanding him to convert to Islam. Khosrau, annoyed that some random Arab merchant would ask him to convert to his guy's religion, told his vassal in Yemen to send two men to kidnap Muhammad and bring him back to...
How about Thomas Dongan, the governor of New York. He did a pretty good job there. Maybe he could have kept the dominion together. I'm not sure how the puritans would react to this Catholicism.
One minor thing. God save the king is actually a french song, under the name "Grand Dieu Sauve le Roi". Here is a link: http://www.youtube.com/watch?v=8FaEqFFAKRA
So the British control Lebanon. That could be interesting. IOTL the British supported the druze, while the french supported the Maronites. I wonder if the french will try to start a Maronite rebellion when the great war starts.
Not massacring catholic missionaries would help. Also you need a nation that will discourage other states from wanting to attack Vietnam. Maybe a stronger China?
I know how to make a Portuguese-Savoy union, but a union with Venice is pretty hard. You would need to Venice a monarchy in the first place to achieve a dynastic union.