Yes, some fans forward stuff to Sam, in the hope of getting noticed and being posted on Central. All Sam does is filter through this stuff and send some on to Morrissey, who ultimately decides what he wants posted on his website. Sam has previously stated that he is not responsible for what gets posted on Central - Moz is in charge.
I think the idea that ALL of this crap comes from fans, and that Morrissey doesn't stumble across some of this stuff himself is pretty naive. Clearly this guy knows how Youtube works, as his into consists of nothing other than YouTube videos. This dancing meme is so outdated and overused that I can easily imagine it was found by Morrissey himself whilst browsing - as with 'cat nodding head whilst listening to The Smiths' it's got a real 'old man on the internet' vibe about it - but yes, it could come from either source.
The ones I think we can be most sure come directly from Moz are those ones where it's 15 different Youtube videos of amateurs covering the same song. I can't imagine anyone other than Morrissey himself being that anal.