That's an interesting hypothesis.
I posted what I originally posted because
the idea of mercurian "rivers of lead" was at
one time a science fiction staple.
But your hypothesis [lake of lead] is equally compelling,
and I agree with you that it's more plausible.
BTW, my desire to know [or hope] if any part of the mercurian
surface is sufficently 'baked' by intense solar heat to a
point where surface dust particles are fused into crusty, fractured slabs or [more likely] into a crusty, fractured 'skin' may be something worth investigating.
Perhaps Arecibo radar data on Mercury could be reexamined
to investigate this possibility.