I'm confused - PLEASE HELP!

Saw this movie for the first time the other night, and there is one thing that's bugging me.....

Bennell says in the greenhouse that once the pods come alive, the original human shrivels or disintegrates. That being the case, it raises the following questions:

1. Why did the human Becky & Bellicec not shrivel/disintegrate when their pods opened their eyes and came alive? Surely when the humans fell asleep (Bellicec in his pool room and Becky in her bed when Bennell goes to pick her up), they would just never wake up again and turn to dust or something?
2. What the hell happened to the Becky and Bellicec pods? Bellicec disappears from the pool table and Becky from the cellar. If they came to life, why did their human counterparts continue to exist? I was watching the whole second half of the movie thinking, 'These 2 are actually pods just PRETENDING to be human, and the big twist at the end will be that they've been pod-people leading Bennell to his own destruction.'

The above is all separate to Becky's 'transformation' at the end, which appears to be a goof by the writers and is a different issue altogether.