Further to my last, and apologies for not reading the final question. The 3 minute rule does, and should exist.
The theory being that if a following lighter aircraft departs from the same intersection as a preceeding heavier aircraft, than it will climb before and above the wake.
If the lighter aircraft departs from a magical distance beyond that of the preceeding heavier aircraft commencing its take off role, than it will be closer to the wake envelope of the preceeding, hence 3 minutes.
I do not want to tell people to suck eggs, just putting forward my understanding of the rules.
Gotta go and have a beer and watch the footy. Cheers!