Am I right in thinking that this should also accept:
My parents have property in Israel
i.e. that נכס can be sufficiently abstract/general to mean 'some property' which might boil down concretely to 2/3 houses, flats etc ?
Not really. Using נכס here would mean that they have one house/flat/any other kind of property. The sentence you suggested would probably be best translated using נדל"ן, which is an uncountable noun.
2019-10-20 That was accepted now.
L-horim sheli yesh nekhes b-israel
As audio says, it's La-horim sheli: "parents" is definite because they are my parents.