I've only taken single-variable calculus, and I'm currently self-studying linear algebra, so I can answer just a part of your questions. Maybe someone who is more experienced will have a better answer, but this is what I think:
I wouldn't take both linear algebra and calculus at the same time because they're completely different, and learning them simultaneously will be harder and probably confusing.
Calculus 2 has to do mainly with integration (well at my college anyways). I'd say it's not much harder than Calculus 1 and probably the hardest thing is making that leap from derivatives to integrals at the beginning. I'm not sure about Calculus 3; all I know is that it has to do with the calculus of vector functions and functions in multiple variables (I think it's like that at most colleges).
I find linear algebra so far to be easier than Cal. 1, but that's probably because I'm only halfway through. I think learning about matrices, determinants and vectors is easier, but beyond that I'm not sure yet.
Hope that helps.