Actual, rather than substituted. Guardians can't give consent for the people in their care. The council can't vote to give it on behalf of someone. Has to be a petition made by the person seeking to have their powers removed.
I don't know. I guess we probably can't enshrine "ask Billy if it's cool" into the constitution. Maybe if someone raises interference concerns, we can have an evidentiary hearing and their squad can vote on whether someone interfered in a way that would vitiate consent, based on the evidence presented.
Age of majority. 18. Unless you've got a better idea.