If you're looking for more concrete answers...
I'd say 3 days for a 1.040 starter wort gravity to ferment out completely,
then minimum of 48 hours at 33F to chill and drop out the yeast before decanting and pitching.
It puzzles me why people say "a day or two" for yeast starters before brew day. That doesn't jive with my experience.
Me thinks they don't chill and decant (what I actually do most of the time) OR they don't really have much experience doing such.
Purely my opinion. Take it for what it is worth.
hen you consider that most of the cell growth occurs early in the cycle, I think its not a big deal if you don't let the yeast ferment out all the sugar in the starter. Does seem like a lot of people pitch the whole starter, without decanting. and pitching a starter at full krausen also seems to have a bit of an advantage although all the regular methods seem to work fine. So I don't think the advice is indicative of someone not knowing what they're doing, they're just doing it a bit differently.
DZ while every time you open a fermentor/starter to the open air does increase the chance of contamination, its not a high likelihood event to begin with. So I think you'll be fine with stepping up as you are doing. Try and do your transfers quickly and in an area without a lot of drafts, this will minimize the amount of dust floating around in the air.