It's been a while, but I usually knock out the eyes with somthing sharp - hammer & screwdriver/awl/nail, and drain the milk. Next i put the drained coconut in a plastic bag or few and swing it against the pavement.  I never had to swing very hard though 1-2 gentle-moderate overhead swings is enough to beat it open.  You get the added bonus of looking something like a caveman while  smashing the coconut too!

About the Thai coconut milk - yes it is supposed to be much creamier than the milk of a regular cocnut - I think they homogonize the milk with some of the meat.  They might add something else, but the only ingredients ever listed are coconut milk, water and some preservative.  I end up using it in thai soups and curries.