1) It's called: e dans l'o. 2) Œ = Alt + 0140 œ = Alt + 0156 3) Only orthography freaks (like me) care about this letter. So I'd say it is safe to just write 'soeur' but I see no point in doing so. It doesn't require much effort, and even if it did you won't have to type it as many times as you will type the vowel 'a'. :)

On a PC, if you switch the keyboard setting to Canadian standard multilingual (Clavier canadien multilingue standard): ctrl + e = œ ctrl + shift + e = Œ I'm come to love this keyboard configuration. It's essentially the same as American English (QWERTY), so there's no real learning curve if that's what you're familiar with (not sure what you use in Brazil, but you write well in English). But it makes typing symbols common to other languages, like é, ç, ñ, ß, ø, €, etc, very simple.

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 ;)