This one felt forced, but maybe that's what helped bring it up to a Friday level of difficulty. Simply removing the letter string "AGE" from words might have proven less interesting. I'm not sure I like the base phrases FIRST MORTGAGE or RAW SEWAGE (for very different reasons), but everything else seems solid. You don't normally see two "letteral" clues in the same puzzle, so even though I picked up on one easily enough (20A: Daffy duo? -> EFS), I must have gotten complacent, because I did *not* see SILENT U coming (12D: Guest letter?). That NE corner has some unfortunate fill, including the oft-(rightly)-derided NLE (19A: Wash. Nats' division) and the Odd Job ALERTER (13D: Siren, for one). I was thinking of the "sirens" that threatened Odysseus and crew, so I went with TEMPTER at first. Most of the rest of the grid (except the far SW, yeesh) seems just fine.
- 17A: Newly certified coroner's assignment? (first morgue) — from "first mortgAGE"
- 24A: Lord's ointment? (noble salve) — from "noble savAGE" (I think I've pronounced that "L" in SALVE my whole life (mostly in my head, as I don't use the word), so this was a surprise!)
- 34A: Mutt with a conscience? (moral cur) — from "moral courAGE"
- 38A: Sitting Bull telling raunchy jokes? (raw Sioux) — from "raw sewAGE"
- 50A: Taxi with no empty seats? (stuffed cab) — from "stuffed cabbAGE"
- 58A: Topping for schnitzel? (Vienna sauce) — from "Vienna sausage"
- 47A: Gulf War reporter Peter (Arnett) — confused him with the "Gunsmoke" actor, Peter ARNESS. Only that guy is actually named James.
- 64A: Bugs once sought by cops (Moran) — not, as you suspected, BUNNY.
- 9D: Migratory African critter (gnu) — once again violating my deeply-held belief that anything that that's bigger than knee-high to me cannot rightly be called a "critter."
- 3D: Candy "whose success is out of this world" (Mars Bar) — this "quoted material" can be found on various Mars websites, but not in any ad campaign I can find. One for Australia/UK (cited at Wikipedia) reads "Out of this world!" ... but in the U.S., no.
[there is nothing about this ad that is not awesome]
- 26D: Breaks up (ends it) — double dose of depressing today; see also 1A: Unceremoniously breaks up with (dumps)
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Everything Else — 1A: Unceremoniously breaks up with (DUMPS); 6A: 1996 film that won Best Original Screenplay (FARGO); 11A: Pro bono TV ad (PSA); 14A: As a friend, to François (EN AMI); 15A: Greg Evans comic strip (LUANN); 16A: Pumpjack output (OIL); 17A: Newly certified coroner's assignment? (FIRST MORGUE); 19A: Wash. Nats' division (NLE); 20A: Daffy duo? (EFS); 21A: Generation (ERA); 22A: In pursuit of (AFTER); 24A: Lord's ointment? (NOBLE SALVE); 29A: Isn't wrong? (AIN'T); 30A: Flood deterrents (DRAINS); 31A: Words spoken with a yawn, perhaps (IT'S LATE); 33A: TV palomino (MR. ED); 34A: Mutt with a conscience? (MORAL CUR); 35A: Annoying negotiator (STALLER); 38A: Sitting Bull telling raunchy jokes? (RAW SIOUX); 42A: Cops may keep them on suspects (TABS); 46A: Rabbitlike rodents (AGOUTIS); 47A: Gulf War reporter Peter (ARNETT); 49A: Peddle (VEND); 50A: Taxi with no empty seats? (STUFFED CAB); 53A: Disappointed postgame comment (I LOST); 55A: Back muscle, for short (LAT); 56A: Nest builder (ANT); 57A: Seventh-largest st. (NEV.); 58A: Topping for schnitzel? (VIENNA SAUCE); 63A: Legal ending (-ESE); 64A: Bugs once sought by cops (MORAN); 65A: Havens (OASES); 66A: Old map inits. (SSR); 67A: Steamed (ANGRY); 68A: Take forcibly (WREST); 1D: Stand up for (DEFEND); 2D: Combat outfit (UNIFORM); 3D: Candy "whose success is out of this world" (MARS BAR); 4D: Downing St. bigwigs (PMS); 5D: Pose (SIT); 6D: Botanist's study (FLORA); 7D: Hearing-related (AURAL); 8D: Wiper (RAG); 9D: Migratory African critter (GNU); 10D: Fit to serve (ONE-A); 11D: Firebird maker (PONTIAC); 12D: Guest letter? (SILENT U); 13D: Siren, for one (ALERTER); 18D: Predicament (MESS); 23D: Pride follower, so they say (FALL); 25D: It's not true (LIE); 26D: Breaks up (ENDS IT); 27D: Baroque stringed instrument (VIOL); 28D: Raison d'__ (ETRE); 32D: "No Exit" dramatist (SARTRE); 34D: Half of MMCXX (MLX); 36D: "What I look forward __ continued immaturity followed by death": Dave Barry (TO IS); 37D: Hung. neighbor (AUST.); 38D: Coulees (RAVINES); 39D: Eternal, and a hint to this puzzle's phonetic theme (AGELESS); 40D: Convinced (WON OVER); 41D: Brewski (SUDS); 43D: Furthermore (AND); 44D: Unsatisfying response to "Why?" (BECAUSE); 45D: Positions (STANCES); 47D: Aqua Velva competitor (AFTA); 48D: Exam given intradermally, for short (TB TEST); 51D: Like the nerve near an arm bone (ULNAR); 52D: Rear (FANNY); 54D: "South Park" rating (TV-MA); 59D: H+, for one (ION); 60D: Work unit (ERG); 61D: Plant (SOW); 62D: Rhine feeder (AAR).