I presume flow will start as soon as you hook up the QD to the gas in post (which is attached at the other end to the top of the fermenter)?
It's been a while since I've done it (having switched back to transferring under pressure), but I think I hooked up the gas tubing first, then the liquid tubing, then just opened up the ball valve to start the flow.
Are you able to cold crash your Brewbucket under CO2 pressure with that ball-lock adapter installed?
Yes. In my experience the Brewbucket will hold 3-4 PSI before gas starts leaking out under the lid. It's also been my experience that it can hold a vacuum. When I cold crash, I pump a few PSI of CO2 into it first, then crash. Before you rack, you need to remember to pump more CO2 in to break the vacuum, otherwise you'll suck in a bunch of air when you open the ball valve.
This is how I attached the gas post (though I think some of the parts links are no longer valid): https://www.homebrewersassociation.org/forum/index.php?topic=20241.0
I've been told by another Brewbucket user that this part also fits: http://www.ebay.ca/itm/232147603644
Though you can't swap between that and a regular airlock or blowoff tube without removing the lid like you can with the stopper assembly. You'd attach a gas disconnect with some tubing that runs into a jar of sanitizer. My only concern with that would be the potential for clogging if you had a lot of blowoff.