1. Can be used instead of "used to" when describing past habits. In such cases, there are no changes in meaning.
I used to eat at that restaurant all the time when I was young.
I would eat at that restaurant all the when I was young.
I am not sure about some of your description of using "used to."
a. I ate at that restaurant all the time when I was young. DOES have a certain change in emphasis, if not in meaning.
b. I used to (or: would) eat there at 6pm. THIS contains definite time.
2. Used in so-called 2nd conditional sentences:
If I studied harder, I would make better grades.
3. There are many other uses of "would" in English, but you asked for the main ones.