Where are you standing and which way is the wind blowing?
Unfortunately marketing has got in the way of some of this,
and also there is often more than one accepted way to define specific drinks depending on where you are, or come from.
However I would like to say that "bucket" sized drinks are not what I think of as proper drinks.
Attempting to answer your first question,
generally a Latte has less foam than a Cap, (and a flat white has almost none). *Latte is commonly described as ~1cm of foam and Cap as 1/3 as you noted, for serving to the public, so you might texture the milk a bit longer for a Cap or you can pour Cap first and Latte second, or you can pour some into another jug first then pour.... etc.
If WBC rules state something different it might just be due to the need to define something more easily measurable so they can more fairly evaluate the contestants offerings.
Rules that state a drink must be served in a "X" sized cup again my be framed for ease/fairness of judging, doesnt necessarily mean the cup has to be full.
My understanding is that many of us here like milk drinks around the 170-220ml range.
Interestingly, if a Cap is equal parts espresso/milk/foam, then ~30ml espresso+ ~30ml milk + ~ 30ml foam is only 90 ml, whereas if you mark equal 1/3 increments up the side of the cup the relative volumes could easily make up 170 or more...which is right?
which do you prefer the taste of?