ad 1)
I have the same problem. When I change high of object in Visitor, it´s look OK in Buldozer, but when I export this island to *.wrp, pack in *.pbo and use in mission editor, all highs are set back to zero. I can´t find solution, please HEEEEEELP.
ad 2)
Yes, it is the "game-metre". All in OFP use metric system.
ad 3)
Try it. Make some terrain on the beath and set highs to -2, -1, 0, 1, 2 meters. Then put some objects at this places (trees for example, "borovice" at -2 m, "briza" at -1 m, "smrk" at 0 m...) and look in Buldozer which objects are in water.