I can't address the mercury issue.
For ventilation, you can add air channels with acoustic labyrinths, one for air going in, one for air going out.
They can be similar.
A simple but effective acoustic labyrinth looks vaguely like a simple periscope; a rectangular tube of some length, say floor to ceiling, with openings on opposite faces at opposite ends. The one for incoming air can have a fan on the regular basement end, say at the bottom of a plywood tube. You could put an air diffuser on the home theater end at the top to spread the air out. Similarly for the one removing air from the theater, but it may not need a fan. It still needs a hole, of course.
The trick is the acoustic lining. Good stuff comprises 1/2" of urethane foam, 1/8" of lead-loaded plastic sheet, and another 1/2" of urethane foam, bonded to the inside faces of the plywood tube with contact cement. A cheaper alternative would substitute ordinary galvanized steel sheet for the lead-loaded plastic. It can be in pieces, without rigid corner connections, i.e., it doesn't have to be a proper duct all by itself; it just has to cover most of each flat surface.
The net inside cross sectional area at any transverse section of the tube should equal or exceed the flow area of the fan. A rectangular section is probably better than a square one, and takes less space away from the actual basement.
If the sound absorption of the tube is insufficient, add baffles parallel to the airflow, comprising galvanized steel sheets with foam on both sides. In the limit, they would form a number of parallel air passages about 1 or 2 inches in transverse/least dimension, resembling a commercial bread slicer with thick soft blades.
Such baffles/grillages are sold commercially for yacht engine rooms, where they allow almost unimpeded flow of combustion air, and suppress engine noise (that's at a nearly painful level) to levels that allow a conversation with ease just outside the bulkhead. They are typically not more than a foot 'thick' in the axial direction.
Sorry, I have no design data for such stuff, so it will have to be cut and try unless someone else comes up with equations or numbers.
Pembroke Pines, FL, USA