Your friendly neighborhood toxicologist chiming in here ...
I have always associated mercury more with urinary excretion, as it has a well-known affinity for the kidneys. But I did some checking, and the ATSDR Toxicological Profile for mercury confirms that it can be excreted in fecal matter as well:
"ATSDR Toxicological Profile for Mercury, Chapter 2"See Section 2.3, Toxicokinetics, which starts on page 133 of the PDF file (the text itself is labeled page 161); see especially page 134/162. For those who want to read more, see also Section 2.3.4 starting on page 157/185.
It's less clear on how effective mercury would be at penetrating the skin, and whether someone who used it topically would excrete significant amounts. However, the above reference does describe a few case studies of toxic effects in people who used mercury-containing skin creams for long periods (see p. 129/157), suggesting that they did accumulate mercury.
If the area where the fecal matter was collected was suspected to have been used as a latrine, urinary or other material might be in the soil at that location. I recall a research project in the American West, studying a suspected campsite of the Lewis + Clark expedition. It was well documented that the expedition used mercury medicinally, and so the researchers tested the soil to see if they could find higher-than-usual levels of mercury.
In this case, the question would be not only whether this particular sample had mercury, but whether it was higher than that of any other islanders/visitors (control samples).
As for lead, I wasn't aware that AE had any sources of high lead exposure--if so, what were they?