1. I know someone who tested this in India.
The UI might be miss leading since there is no indication that colonies RGO are less productive. I'll make my own tests when I have some time to spare.
2. Japan should have enough score to be able to get enough raw materials for its factories from the world market.
Not if there is a shortage as it is often the case in the late game.
3. If your uncivilized and already having crises as Japan, you're doing something wrong. Early westernized Japan can stand its own against Russia because of the piece- meal way the AI deploys its armies.
As long as you are not a GP, you can loose the territory without a fight. I will concede that it generally don't take long but it is still a possibility. Also, conquering Korea as an unciv combined with the westernization modifier and the events related to it can increase militancy are more than enough to create tension and possibly a crisis.
4. Korea is a better bet than taking China, Indochina, or Indonesia. Those are worse ways to use infamy than taking Korea.
China produce a lots of tea, silk, iron, coal and have a high population.
Indochina produce rubber.
Indonesia produce oil and rubber.
5. Not if you have high level naval bases. If not, then you might need to waste 5 infamy taking a single state from an unciv.
Depending on your luck and how quickly you westernize, you might not have that many naval doctrine tech. Constructing naval bases also take a very long time so getting high level naval bases might not be an option. We agree about the unciv conquest approch.
7. This betrays your ignorance. Cores only give a -20% malus to assimilation. The Ultimate Mod has something that gives +60% to assimilation. Pops start assimilating even if they're the primary culture of a nation with cores there. You can go test it out.
There is no way to get +20,1% assimilation ingame. Saying it was impossible in vanilla is therefore correct.