The only problem I see with creating a colony is that you will lose your presence there. What happens if you are fighting an overseas war? Will your new beloved vassal be any help?
Your vassal will declare war upon your enemies when you do. If they'll be any help will depend from time to time. It'll depend on their power, their distance from the battle ground, and on the war-eagerness of the colony. (as with normal vassals)
When you create a colony, do they have your exact technology level?
yes
Colonies start out loving you, but some civs are backstabbers. What are the tendancies of a colony to backstab you?
A colony must break free first, then it must at least wait 10 turns before it can declare war upon you.
Since colonies start friendly with you, they'll not easily break free anyway.
So if you have a corporate branch (say, Ethanol) in a city, and that city is cut off from any source of corn, do you still pay maintenance for that corporate branch?
No
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