No, but don't let that stop you
Ever since the release of the topographical maps of Mercury in 2016 (see also this), we've learned a lot about the surface of Mercury. There's not a lot of metal on the surface. 1. What analysis we've performed doesn't seem to indicate the presence of liquid metal lakes.
But that doesn't mean you can't have them. Let's investigate!
Mercury has more iron in its core (by volume) than Earth. 2. That means that your premise is plausible. What we need is some volcanism to push some of that iron-rich core to the surface.
Turns out we know a bit about volcanism on Mercury.3 I honest don't worry about the specifics other than it happened a long time ago. That's good! It means there's been time for liquid iron to be pushed to the surface and then be covered by a layer of dirt to form a crust.
Does this require suspension of disbelief? You bet. Millions of years of dirt blowing over a liquid anything would basically mean you have to mine to get it, not fall through the crust. Worse, it might be liquid on the day side, but it's not liquid on the night side. Mercury's days are about 58.5 Earth days long. That's plenty of time to melt the metal (assuming it's reasonably exposed and not insulated) during the day — but it's also enough time to reasonably solidify it over night.
So, while reality tells us that there are no liquid metal lakes on Mercury, I believe there's enough circumstantial evidence to justify the idea from a suspension-of-disbelief point of view.
As for your second question (please remember in the future that you're expected to ask only one question per post), pressure variation has little affect on melting points, but there are some ideas to work with:
- The pressure does have a very small effect on the solid/liquid
equilibrium. For water, going from atmospheric pressure to near a vacuum
raises the melting temperature by about 0.01 K. For most other
substances (where the solid is more dense than the liquid) the effect is
in the reverse direction (so that vacuum will lower the melting point),
but the effect is still small, at least for the small pressure difference
between atmospheric and vacuum. For the explanation of this effect, look
in a physical chemistry textbook under "Clapeyron equation."<
- For some substances (especially metals), air can react with the solid
and form a compound that will have a different melting point than the pure
solid. This could affect the melting point in either direction, and doing
it in a vacuum would get rid of this effect. Some high-temperature work
with metals is done in a vacuum for this reason.
- Solids can also "boil" since they have a vapor pressure (this is called
sublimation). So, for example, if you make vacuum above some ice, it will
eventually sublime and go away (like snow can evaporate on a dry sunny day
even if the temperature is below freezing). But this is not melting.
- For some compounds, if you heat them they will decompose before they
melt. If you are under vacuum, the products of the decomposition will get
carried away faster, so the substance will disappear faster. But this is
not true melting either.