Wenn Sie Fragen haben, lassen Sie es mich wissen.
Why do we use es here? As Fragen is plural, why don't we use sie?
Whenever I don't want to repeat the noun, I either use its gender article as per the case or the corresponding pronoun for it. So, could the following be considered valid, too?
Wenn Sie Fragen haben, lassen Sie sie/die mich wissen.
Fragen is an abstract word. Instead of Fragen, I could also use another object, e.g., der Stift:
Das ist der Stift. Ich benutze ihn/den zum Schreiben.
Am I following the right path?
While reading I have noticed that some examples show two clauses using a relative pronoun and some examples show two different sentences using a relative pronoun.