1) - Do they still use lead-containing gasoline in Portugal?  If so, it
seems reasonable that lead-containing deposits from automobile exhaust might
well contaminate roadside vegetation, which theoretically could become
concentrated in the tissues of snails which feed on that vegetation.  I
don't know if any actual studies have been done to establish this. It would
be an interesting question to explore.  I have seen warnings against
gathering edible plants from such areas, so I would be suspicious of snails
feeding on those same plants. If the snails are lead-contaminated, no manner
of cooking will remove the metal.

2) -  I know of no normal human parasite that uses land snails as an
intermediate host, though there are plenty of fresh water snails that serve
as vectors of human parasites.  By "normal" I mean parasites whose usual
final host is humans. There are some very rare cases of bovine lungworm
infection in humans, and certain terrestrial slugs and snails do serve as
intermediate hosts for such parasites.  Cows get infected by inadvertently
consuming snails along with the grass they eat.  But those snails, while I
don't remember the exact genera involved, are tiny species, not the larger
forms usually eaten by humans.  And, to become infected with anything from a
snail, you would have to eat the snail uncooked.

Paul M.