Sometimes I see the wer/was relative clause followed by der, dem, and den. But sometimes it's not. For example (excerpted from here):
Wen ihr einladet, den müsst ihr auch empfangen.
Wen ihr einladet, müsst ihr auch empfangen.
Looks like "den" can be taken out because we can identify the subject (ihr) and object (wen ihr einladet) and the sentence can be comprehensible without it. But I'm not sure. That's just my guess.
The reason why I'm not sure is that every example in my textbook has der/dem/den after the w- clause (wer.../was...):
Wessen Herz rein ist, den habe ich lieb.
Wem du geholfen hast, der hilft dir auch.
Wen ich ehren möchte, der ist er nicht.
Wer den ganzen Tag arbeitet, der ist abends müde.
If the distinction between the subject and the object is clear-cut, is it always allowed to leave out the demonstrative pronouns ("den" and "der" in the above cases) after the wer/was clause?
Also, will the sentence below make the reader confused about who does what, without der or dem? (I mean, I wonder if this sentence can be interpreted in different ways without der or dem):
Wen ich gestern getroffen habe, schenkt ihr ein teures Bild.