hmm, yeah ive thought about the tremor being luke and vader just not realizing it. And Luke was in close proximity to vader before that (he wasnt that far away on the death star, and vader was right behind him on the trench run, but Luke's lack of experience (in the force) could be attributed to that, because Vader did sense how strong the force was after Kenobi spoke to Luke (also can assume vader didnt really know about this speaking from beyond thing)

still doesnt address the twins not having an umbilical cord though

-Darth Simon