the He valve may screw/unscrew but does not go through the entire case-theres no hole through to the movement so no probs there, the crown screws down pretty good (on mine atleast) with a lot of turns to lock it, crystal gasket was a little thin when I stripped my entire watch to check the build but works fine, caseback gasket is present and if greased will be fine, caseback tightens down nice and tight...and to top it off, I had mine pressure tested to both 50m then 100m at a local shop-it passed no problems! ofcourse get yours pressure tested as not all reps are the same...
the watch is bigger than a sub c yes...I found my no date sub c rep wore very small, good looking, but the seamaster feels more substantial on my wrist...