I would say that perfect information is not necessary for legitimate consent, per se.  But all parties must honestly impart all pertinent data They have.  Withholding pertinent data, or delivering it in convoluted ways (like legal mumbo jumbo in find print) creates a non-consent situation.

Yes, when all data available is on the table, One can legitimately consent.  Even if there are missing data neither side (or none of the sides) has.

Duress is when one side makes it clear They will create problems of some kind for the Other if the Other does not agree to terms.  This is not to say that conditions from outside the agreement cannot be a consideration, but "duress" is willful.

Nothing in the pirate system that came on land literally through the sewers is genuinely consensual.  They whole system is founded on hiding things.  If Ethics are upheld (the three Laws - the only Laws - are upheld), then the legal system is irrelevant.  And if they are not upheld, there is no legitimacy.

Define "imperfect circumstances..."

As for principles, if Ethics (the three Laws) are upheld, then they are indeed princilpes.  if the Laws are not, We have preference.

And not all of Us abandon Ethics when inconvenient...  I could steal and rob and have a much more convenient life than I do.  But I am supremely Ethical and will not do that.