1) There's no specific name for this ligature of o-e.
2) French keyboard or not, you won't find the touch "œ" in direct access. If you have a mac, just hold the "o" touch, and it's gonna appear.
3) Theoretically, a formal text must be perfect so yes should be an error, but a lot of french people use to make mistakes about this letter so don't worry about that ;)