Note: This is another in my sequence of post-Mystery-Hunt cryptics, which started with "Still There," "Post-Game Predictions," "The Sequel," "Power Lines," "A Lesson in Cryptography," "Yet More Villainy," "Further Escapades," "Time X*X", and "Honeymoon in Segas"; this one was written before the 2013 hunt and refers to the 2012 hunt. (The tags [NI2] and [NI3], as is usual in the National Puzzlers' League, indicate that an answer is in the 2nd/3rd edition of Merriam-Webster's New International Dictionary.)

Leo: Max! We won the Tony! Do you know what that means?

Max: Of course! It means we'll get tons of investors for our next show! So if we sell 100,000% of it, and then really guarantee that we have a sure-fire flop . . .

Leo: [Shrieks.]

Max: No, no, this time it'll work, because instead of just tossing a couple of off-putting things into the show, our expert dramaturges will be working like crazy to ensure that the music, choreography, and book are all terrible. Look, I've written out the entire plan in the form of a cryptic crossword. Seventeen of the Across clues are (alternating) Song and Dance clues, in each of which an appropriate two-to-four-letter sequence must be removed from one word before solving.

Leo: Hmm . . . the sequences from the Song clues seem familiar . . .

Max: Well, they should. If you need information about directionality, any Dance-clue sequence above the neck means up and any below the hips means down. Also, for each Song or Dance clue, where the nth word is the altered one, take the nth letter of the clue; the letters from the Song clues will spell out a Bad Thing that might happen with the songs for a show, and likewise for the Dance clues and dancing.

Leo: And will these Bad Things actually happen?

Max: They certainly will! There are also (in the proper quantities, but not strictly alternating) Song and Dance Down clues, all of which work normally. For each of these, the answer must be altered before entry (in six cases this yields a proper noun) so that the Bad Thing affects a letter sequence (for a Song answer) or single letter (for a Dance answer). (Any tagging on clues applies to answers before alteration.) From top to bottom in the grid, read the initial letters of the affected sequences for Song answers, and (separately) the affected letters for Dance answers, to find out more specifically what the Bad Things are.

Leo: But what about the book? What's wrong with the book?

Max: Well, dialogue is just an exchange of words, right? So the five remaining Across clues clue the five remaining Down words (in no particular order) and the corresponding Down clues almost clue the corresponding Across words; if you read the omissions from top to bottom in the grid you will find out what has been left out of the book.

Leo: Got it. So who are these expert dramaturges you mentioned?

Max: To get their names, just read left-to-right in the grid every phrase you just read top-to-bottom. Or write the first letter of every book clue beside the clue it exchanges answers with to get a description of the entire set.

ACROSS

1. Avians that can't loft her weird son will take solace
4. Quilt that's brownest originally, sported by poet William Cross
10. Tailor's novella, portraying a petroleum byproduct (2 wds.) [NI2]
12. Elk chilled dessert via atmospheric vibrations
13. Grant legal status to Tesla, then stick around
15. Lance almost slipped inside of barn
17. Number of sides on the dodecagon-shaped salt or diamonds, e.g. (abbr.)
18. Jawa city's Drag Night's beginning
19. Headgear abandonded by milady after some bids for attention
23. Urchin beheaded James, who sang loudly about being less skinny
24. Imitators of solaria, at first, upset congressman with scruple
25. Mr. Hammer from The Social Network and Legendless Legions
27. Successor to nouveau chéri dances
29. Connecticut holds mired, gutted Isuzus for ex-governor Charlie of Florida
30. Are rife with people who do not play searingly for an audience
33. Freezing cold lagoon's heading east near Fez, perhaps
36. Aimless, lost, limp machine in a small area (hyph.) [NI3]
37. West Side Story character with numbers echoing tribune
38. English leader of Iceland obstructs Latin Star Wars game, perhaps (hyph.)
39. Shindigs unearth these sodas around Yale
40. Odd parts of sheaths defend developed ovules in fruit or consoles (2 wds.)
41. Said grayish horse performed

DOWN

1. Like Roosevelt, hug a Texan athlete
2. Flower child is apt to spring back, abandoning work
3. A land, essentially, that ultimately sunk into rising muddy stuff!
5. Outspoken people who sack string players [NI2]
6. Andromeda actress Doig's quartet of flexagons
7. G-man Eliot's directions for writing a reverse gamma?
8. Units indicating purity of over-harsh, grasping adult
9. Those who try to start titrating -CO-O- compounds
11. Nixies at last cross gyrating portals
14. Suddenly say, "Mr. Bacharach has left"
16. What a couple from Tarentum describing Earth, by rights, should say!
20. Lieutenant inside may like beer
21. I'm an unusual Ugandan dictator
22. Using your mouth, chew on leaves like a bad boy
23. Possible homes for coral snake finally captured by umps
26. It's about lines the author ran across, with unfortunate consequences (hyph.) [NI2]
27. College official east of Boston that is Mark Wahlberg's brother
28. Ancient Italian ring's tin
31. Arwen's people will claim 25% of Isildur's people like the King
32. Mot preceding majesté — or else, malade
34. Acoustically detect water in Cuba? Yes, indeed (2 wds.)
35. Chimney component traveled by air, reportedly