>Is lactose able to pass through the breastmilk?<

It's pretty highly unlikely that any intact molecules of lactose 
would make it through the villi of the intestine to enter into the 
bloodstream in the first place.  But since human milk contains 
the highest percentge of lactose in the animal kingdom 
(about 7.0% on average), I don't think it really matters much 
if a few molecules do slip by.

Steve Carper