This was one tough puzzle. I had to walk away from it a few times...and had to Google a few things.
Many of my answers were nothing more than guesses that panned out...including:
16A: Compulsive shopper (oniomaniac). I didn't know there was a word for it...although I should. I'm a recovering oniomaniac. It's been more than a year since I found bags of clothes bearing their tags on the closet floor.
18A: 20th-century scandal (Teapot Dome). I must have heard it at one time in my life...or maybe nothing else would fit.
26A: Birthplace of Sert and Miró (Barcelona).
29A: "A Clockwork Orange" instrument (moog).
36A: Dawn observance (matins)...couldn't remember if it had an I or an E.
38A: Folds (plies). The ballet move would have been an easier clue, but it is Saturday.
39A: Giant perissodactyls (rhinoceri). That's probably never been in a NYT puzzle.
50A: Mixture (salmagundi). I was clueless and only got it from crosses...but still thought it must be wrong.
2D: "The Last Don" sequel (Omerta). I thought it was "The Last Don II."
8D: City whose name is Siouan for "a good place to grow potatoes" (Topeka). I didn't know that's what it meant, but with the K in place, it was a very good guess. I'm thankful I knew 24A: Product once advertised as "Ice-cold sunshine" (Coke). Sometimes it pays to be old.
13D: Flattering courtier who changed places with the tyrant Dionysius, in Greek legend (Damocles).
32D: Defensive structure (palisade).
33D: Person not easily budged (stickler). Love the word...maybe because I am one at times.
There was so much I didn't know in this one. The crosses helped me get 5A: "L'Orfeo" composer (Monteverdi), 30A: "La Bohème" setting (garret), 31A: "Casablanca" screenwriter Julius or Philip (Epstein), 55A: Lassie creator Knight (Eric), 4D: Boxing-related (fistic), 7D: "Love Jones" actress, 1997 (Nia Long), 12D: Western Sahara region (Rio de Oro), 20D: Only starting pitcher since 1971 to win a league M.V.P. award (Clemens), 27D: "Such Good Friends" novelist Gould (Lois), 28D: Writer of the story upon which "All About Eve" is based (Mary Orr), and 30D: "Treasurer Island" character (Gunn).
Favorite clues include 1A: Lock combination? (coif), 15A: It means everything (omni), 17A: Seeks change? (begs), 21A: Court calls (lets), 25A: Bicycle pack (deck)...one of just a few gimmes, 46A: Like M, L or XL (Roman), 56A: Windy? (serpentine)...a long I sound when I was thinking short I, 57A: Check for letters (rent), 1D: Network seen in many homes, and not proudly (cobweb), 9D: Engage in cabotinage (emote), 14D: Blade holder (ice skate), and 31D: What "!" provides (emphasis)...another gimme.
There are a few answers that I can't make any sense of. I just checked Harris's grid at Water and Waves, and the answers are right. They still make no sense. Can anyone explain 25D: Hamlet (dorp), 37D: It could end up in a fiasco (chianti) and/or 51D: Relief provider, maybe (map). It seems as though there are more, but I'm having trouble keeping my eyes open any longer.
With that, I'm calling it quits for this one. Here's the grid...
...and I'll see you tomorrow.