There definitely is a lot of '60s riffing on the album, but I don't get the sense it's mockery - more homage (the backing vox on "In The Flesh" are perfect, for example) with a bit of a cool, standoffish affect (kind of Harry's default vocal style). The CD version has a bonus track cover of the Shangri-Las' "Out in the Streets" that's pretty faithful (they covered it again on No Exit).
I can't really argue with you on Plastic Letters. It's definitely the least obviously melodic of their classic albums. The pleasure of it for me is the weird off-kilter nature of stuff like "Bermuda Triangle Blues" and "Contact in Red Square" - the structures are so odd. Maybe this is the album where they feel most like a band? It feels like proto-Guided By Voices to me at times. That said, looking at the tracklist, the second side is kind of weak, although I do like "Cautious Lip" a lot (its riff is maybe the catchiest thing on the album). I guess it's kind of an album full of good filler songs without any singles, but I love good filler.
I loved SF Sorrow as a teen and still enjoy it - it's one of the best-produced albums I know. And yeah, "Walking Through My Dreams" is a banger.