I've been using "both top and bottom entrances" on a couple hives for five seasons now. Many (most?) ferrel colonies also have two entrances. I'm convinced in its effectivness in helping to keep the hive inside dry. Also use SBB with the trays in place on all my hives but one (last experiment). As for the white specks; Pure, new wax will taste very sweet. That said, Its always a mystery (to me) what the bees decide is trash and what is not sometimes. In the spring, before anything has begun blooming around here, even if I've fed, my bees are all over the old cat litter I've dump around the garden perimeter all winter, rolling in it, seemingly playing, and for whatever reason, taking some of it back to the hive. Hasn't hurt them yet as far as I know, its a "corn" based product though, so that does concern me with all that's been written about same.
One of my favorite aspects of keeping bees is still the mystery of it all.