I believe the A&D arm was designed by John Wright, who went on to found IMF. It was a well regarded arm, though somewhat ahead of its time, since its wireless contact design was close to frictionless, and the very high compliance cartridges that needed such low friction were mostly in the offing, though Peter Pritchard's ADC cartridges were pushing the envelope early on. Keith Monks bought the design, and at some point, switched from stainless steel pins and contacts to cheaper brass or bronze, not realizing that mercury dissolves copper, so the later arms were prone to having the pins that transferred the signal collapse. Worse still, the contacts at the bottom of the mercury well also dissolved, and often mercury spilled out all over the place.
It is true that elemental mercury is actually relatively nontoxic, but mercury salts (and mercury vapor) are very toxic, and who wants to have mercury coming in contact with other chemicals it can react with?
A&D arms ought to be perfectly safe as long as the arm stays upright, but check to make sure there isn't mercury slopping around in the bottom of the plinth -- I would do a very careful check of the path of the turntable into your house also, and check the local university chemistry department for how to clean up mercury -- I think you usually use sulfur dust.
But be discreet -- people are not any good at assessing risk, and even though your car is a vastly greater health risk than elemental mercury, you don't want your house becoming an EPA hazardous waste site. I say this as a strong supporter of environmental regulations, but one aware of how one-size-fits all some regulations can be.