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Is this a single tank rig?
If you have to "squeeze" and gyrate to empty the BC, I would say look look at another wing.
You don't. All air will escape if you are vertical, but you have to hold the deflator hose straight up. If you leave a bend in it, you will pocket air. You cannot deflate it all the way if you are horizontal. Slightly nose down, pull your two bottom purges.