Most of your ideas are sound. It really depends on how black is playing, in the nimzo Indian an early Qc2 has it's benefits, others like say, the slav or QGD... meh, not so much. Just stay flexible and don't develop the queen early unless you have a very concrete reason.
Queen placement with White in systems with d4 and c4
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I sometimes find myself wondering where to place my queen with White in d4 openings.
Are there any good general rules or abstractions that could help me here?
My current approach is this:
Put it on c2 most of the time. It supports the c4 pawn (especially in Catalan this can be important) and can support an eventual e4 push. Put it on d2 if I have a bishop on the c1-h6 diagonal and I want to exchange his counterpart on g7. Put it on b3 if Black developed the light-squared bishop early so I have some pressure on b7 and maybe d5. a4 only if I desperately need a check to recapture in c4 or need an extra attacker after putting pressure on a c6 knight with Bb5.
Of course there are always concrete exceptions, but even from my general scheme I sense that there could be improvement.
Which piece exchanges favour which square (for example the BxNf6 exchange, one an exchange of a pair of equivalent minor pieces)?
I know my question is very general and might be hard to answer, I'd still appreciate some advice.