sharpdion. I don't mean to be rude or anything but, all your questions are looking for answers.
Most of the time there are none. Bottom line is, do what sounds good to you.
And, I don't mean to say the responders are wrong.
The trick is, putting in the time to know in your head, heart and ears what something will sound like.
In other words, you hear something in your head, you know what to reach for to achieve it.
As for your question about chorus, "use the chorus fx to make a choir effect". If you put a chorus on something, it will sound like you put a chorus on it or not, depending how you mix it in and the settings of the chorus that you use. It can be useful in any situation or not. Especially depending on what you are trying to achieve.
All that said, I think that whatever questions you have and whatever answers you get, I think it is most important to consider the style that you are going for. That to me is the deciding factor in most decisions if you want to put it into a "this does this and this does that" kind of thing. As the doubling a guitar thing goes, there are certain styles that this works great for and some not so much. And, it matters much as to what level/volume any effect is at. Is it supportive or just an all out effect?
I don't know if I'm getting across what I'm trying to say and I should probably just delete this.
But, oh well. That's all I got.