This puzzle was originally formatted as a PDF; the puzzle PDF is available on Dropbox. A screenshot of the PDF is available below. This puzzle is an entry in Fortnightly Topic Challenge 46, "Tales from the Cryptic".
This is the third puzzle in a sequence, coming after Not Bluebeard's Castle and Oops! All Acros--. The sequence is themed after a certain set of works, revealed in the first puzzle. No knowledge of the works in question is necessary to solve this puzzle. It is highly recommended (though not necessary) that you solve those two puzzles before this one.
This puzzle contains a few crude references; solve at your own discretion.
Transcription of the instructions and clues:
In the third part of this four-part series, we see two different perspectives on the same work: one small version in which things go as planned, and one large version in which they do not. [boxed; right side up] nine of this side’s entries are too small to fit in the grid – you’ll need to stretch one letter between two cells. if you stretch the nth letter of the answer, highlight the nth letter of the clue. the stretched letters, from left to right, will spell what the cast hoped to find on the missing page. [boxed; upside down] NINE OF THIS SIDE’S ENTRIES ARE TOO LARGE TO FIT IN THE GRID – YOU’LL NEED TO SQUEEZE TWO LETTERS (OR IN ONE CASE, ONE LETTER AND ONE SYMBOL) INTO A CELL. IF THE NTH CELL IS THE DOUBLE-PACKED ONE, HIGHLIGHT THE NTH LETTER OF THE CLUE. THE DOUBLE-PACKED LETTERS, FROM LEFT TO RIGHT, WILL SPELL WHAT HAPPENED INSTEAD. The highlighted letters will pair up in corresponding clues – they give you part of a cipher that converts between small and large letters. The remaining 26 clues each have one word that is the 'wrong case': to be able to solve the clues, you must convert that word to the other case with this mapping. The stretched letters, when converted to their larger counterparts, will tell you what happened to the missing page.
across 1. cyber-nations' golden fruit conceals one fay crystal at its center 12. mane relative is not in favor of 13. "gaze and add forevermore", poetically 14. metal slab in the front 15. in french article, he created legends 16. nobleman caned, losing face 17. native to some of rome 19. face of soldierly nation creates type of tank 20. add more of a vista to newly-created seal (2 wds.) down 1. device that's boring and totally sound 2. for chum in the uk, ton of fiddlers 3. ignore lack of force in revolt territory 4. lysed borreliosis extract for someone in edmonton 5. huggable bartender 6. frank dich's army unit ate before lunch 7. upset lowlife sailor 8. inheritances missing preface can lead to problems 9. confesses on supple, poorly-written, dismissed letter on disks (2 wds.) 10. timberwolves' group in cycle stop 11. swallow carrying ten of a headpiece ACROSS 1. FEW TO LEAVE ENGLISH HEIST, OUT IN SOUTHEAST (2 WDS.) 12. CAUSE BLUSH IN SOMEONE BEFORE TUESDAY 13. ONE HOLDING TEAL COMIC UPSIDE-DOWN, HEADING OUT 14. HELPED SOUP PRODUCTION 15. ICY VET FROM A DISNEY MOVIE PEELS AWAY HIDES 16. GRAB ONE PART AT MIDDAY 17. ABOARD HELPLESS YACHT IS DEFENDANT WHO WAS DECLARED GUILTY 18. ASSHOLES ARE REAL DRY AROUND CONNECTICUT 19. EMPTIER VESSEL THAT HOLDS A LITER 20. SECOND-GENERATION JAPANESE IMMIGRANT'S SOUTHEASTERN DELI NO. 2 DOWN 1. MILITARY CAMP TO LISTEN IN, PROBING EMPLOYEE ON VACATION 2. WHAT GOES UNDER A "DEAR...": "SON, I'M GOING TO..." 3. CAUGHT SANTA'S ADDENDUM 4. FORMAT LAST IDEA FOR BOOK 5. AVOID, LIKE BEING IN LOVE WITH TELEKINETIC 6. CHRONICLE LOOKING BACK OVER APPRENTICE ARYL OF THE JETSONS 7. GIVE A SECOND CHANCE TO BREAK IN, PRE-DOPING 8. FOUNTAIN NYMPH ORACLE EVACUATED? THERE ISN'T ANY 9. CLEARS OUT REPO #3 OF STUDENT APP CAPTCHA KEYS 10. AUTHOR CANAVAN TO REWRITE UNFORTUNATE DEATH INVOLVING CHARACTERS IN THE INTRODUCTION 11. SUCCESSFULLY SEIZING ANNE'S POORLY-BUILT GIRDLE 18. NEVADA CITY LINES L-BAR RIGHTFULLY ELEVATED