"The main problem with rivers of Lead, is that you need a process to return the lead to the highlands again. SO you'd need an atmosphere, clouds, rain/snow of lead."

Lead. Melting Point: 327.6 C Boiling Point: 1740°C
http://environmentalchemistry.com/yogi/periodic/Pb.html

Don't know lead's boiling point in a vacuum, but it would be less than given above.

The highest surface temps on Mercury I've seen cited are:
"447 deg. C", "almost 500 degrees C", and
"870° F (466° C)"

For comparison, Moon's highest cited temperature is 123 C.
On Earth, it's dependent upon the type of material exposed
to sunlight [as it would be on the Moon, Mercury or elsewhere].
Man made roofs or roads it can typically get to about 80 C, but natural terrain can become fairly hot too:
"In the Kalahari Gemsbok park, where the most extensive records exist, a maximum of day temperature of 42 deg. Celcius was measured in the shade. Under these conditions, the temperature on open sand can become as high as 70 Deg. Celcius.
A few centimetres below this, the sand temperature may be 20 to 30 degrees lower...."
http://abbott-infotech.co.za/kalahari%20desert%20climate.html

Modern humans seem to lack interest in the temperature of surfaces they could walk on. Exceptions to this can found when some primitive people "fire walk", and there is some interest in cooking eggs on sidewalks.

Though it seems likely that lead could melt on the Mercury surface, I'm not certain it would.

Say you had a sheet of lead, and you put it on this sand which gets up to 70 C in the sunlight- does the surface of the lead heat up to a higher temperature than the sand surrounding it?

If you did this with a sheet of iron, I would think the iron might reach a higher temperatures, but I'm less familiar in regards to lead.

And of course different metal alloys or different metal ores would have different thermal characteristics and different melting temperatures. And one has different thermal characteristics in terms size [and shape]- micro particles to boulders.