Fireplaces, Heating Stoves, Flues and Chimneys - fireplace / chimney /weird vent question

ok, i have a very old house with one chimney. it seems to
have two "halves" one for the furnace and hot water heater
and one that I surmise was once for wood stoves. It seems
to have a hole in it on the kitchen side, and one on the
dining rooms side, directly opposite the hole in the kitchen
side. there is also a hole upstairs in the bedroom, which
has been bricked over. these holes are about 6" in diameter,
which looks like the right size for a stove pipe.

Here are my questions:

1) can I install a freestanding fireplace into this chimney?
if you are not sure, what do I need to look for?

2) there is a very strange vent in the upstairs bedroom. this vent
is metal, and goes straight into the chimney. it does draw
some air, so I assume that the chimney is pulling some air
out of the house. what is this for? can I remove it safely?
is it safe to leave it? it seems as if it would let smoke into
the house if I installed a fireplace!!!

trance,

At one time there must had been free standing stoves in the home. I recommend that a professional look at what you have before proceding any further to ensure that safety measures are followed to avoid any problems.

Good Luck!