I’m not sure about your first topic. No experience.
In regards to your second topic, gravity filling a mold from a
crucible on top, this would work, and is a method not uncommon in the
casting industries (See the photo on this web page:
http://tinyurl.com/25n8947). The mold has to be kept warm so that
the metal won’t freeze upon entry. In the jewelry industry this is
commonly a vacuum casting machine with an electric induction heated
crucible above the warmed mold, with the crucible and mold enclosed.
There is a gate between the crucible and mold. A vacuum is created in
the mold, and when the metal is ready the gate is opened and gravity
and the vacuum draws it into the mold.
I’m sure it worked by gravity alone in the days before we could
induce vacuums, but you’d still need to warm the mold to ensure metal
flow to all parts of the mold.
Mike DeBurgh, GJG