Back in my early 70's naval architectural days I cast a dozen "ducks", or spline weights out of lead in a couple of ordinary plaster of Paris molds. The molds were cured for a week first (which may be the missing element here) before baking in the oven. I don't remember them steaming at all despite being unofficial non-investment, and the castings of about 3-4 lbs each came out well detailed. The molds were re-used an average of 6 times ea., so I'm sure it can be done.
Lead casts at about half the temperature of aluminum, and can be melted, as I did, on a camp stove outdoors. Fumes are dangerous, and lead is a cumulative poison in the body, of course, and this should only be done outside, and with intelligent precautions against inhalation and burns.
Vaseline, grease, butter, lard, etc all work fine as release agents for plaster mold cast over an existing part, or a carved wooden plug or pattern (as I used for my "ducks") -- as long as draft is maintained. And the results will look a lot better with a grease than cling wrap. Just some tips for next time.
I think the use of an old cheap sawed off hammer was very clever, and I'm sure you could clean up that head if you wanted. Or actually just re-melt it and cast anew for a better look, if important.