Did you treat for Varroa?  If so, how?  Bees die of varroa in very much the
way you described, leaving behind plenty of honey, and a hive with very few
dead bees.
 
With varroa, you should be fine reintroducing a package in April.  Research
indicates that varroa can live on dead bees for a couple months (I don't
remember exactly how long), but April is plenty long enough.
 
If you have foulbrood, there should be dead capped brood in the hive.  If
there is no dead brood, or very little (a hand-sized area=very little), you
probably didn't lose the hive to foulbrood.  If there is dead capped brood,
and the cappings look sunken and oily, and have holes chewed through them, be
suspicious.  Open up the cells, if the dead larvae looks more like
butterscotch pudding than a bug, be more suspicious.  Hold the frame at an
angle so you can look at the bottom of the cells (the gravitational bottom,
not the bottom where the queen lays her eggs).  If a lot of cells seem to
have a thin dark-colored scale in them, you have foulbrood.  Burn the frames
with the combs and scorch the inside of the boxes.  You can start a new
colony in the boxes, on new frames.
 
Cold alone will not generally kill bees.  Bees do starve to death if they are
too cold to move over to a new patch of honey, but then there will be a solid
clump of bees in the hive, huddled over an area of comb where you can see
bees dead, head-first in cells, looking for a last drop of honey.
 
Mold is also not an indicator of anything, except moisture.  Generally,
however, a healthy hive will keep air moving in a hive enough to prevent mold
from growing inside.
 
Do you get freezing weather between now and April?  If so, leave the hive
alone.  Nothing much will happen between now and April.  You can try putting
the hive bodies in plastic garbage bags, if you don't want to leave them be.
 
Frankly, I think you had varroa.  Right now, the only legal way to keep bees
alive is Apistan.  Get some, and treat according to the directions on the
package, unless you want to get new bees every year.  I have found that I
need to treat twice a year (spring and fall) to prevent a nasty population
crash.  But we're in California, and we get re-infested every spring in the
almonds.
 
Good luck,
  Shawna Roberts
  Gypsy Bees