A: For the most part, fair use likely protects data collection for the purpose of research. This just means that it's not something additional you need to worry about on topic of TOS violations or ethical considerations. [1/2]
A: Super tough ethical questions here, in part because of tensions between research ethics and open science. (I have a lot of thoughts on this, I should blog.) Consider just as you would for collecting data. [1/2]
A: In many cases I think you can find a way to do research ethically. Thinking through ethical considerations is about finding ways to mitigate harm-- [1/2]
A: YES YES YES. Ethical consideration in data collection should be part of the methods section. Explain why you did what you did. This is how we form community norms and best practices. (I do this when I use public data!)
A: The answer to this is 'it's complicated.' Unfortunately this isn't really a settled issue. I go into a lot of detail in the paper about the current legal landscape, so I'm going to refer you to that! (Though IANYL ;) )