My solution of this puzzle, first published in the i newspaper on 22 August 2020.
The message around the outside of the grid was THINKING OUTSIDE THE GRID (starting in SW corner and proceeding clockwise). The extra letters spelled out SOUTH EAST and NORTH.
The nine blank cells were at positions 3, 6 and 9 in each of rows 3, 6 and 9. The added cells were at the top and bottom of columns 1, 3, 5, 7, 9 and 11; and at the left and right of rows 1, 3, 6, 9 and 11.
The line I drew started in the SE corner and went diagonally up to the blank cell at row 3, column 3; then rightwards to the added cell at the end of row 3; then diagonally downwards to the added cell at the bottom of column 3; and finally straight upwards to the top of column 3.
[Edit 7/9/20: the published solution was different from this, starting in the SE blank cell and then proceeding NW to (3,3); S to the bottom of column 3; NE to the right end of row 3; and W to the blank cell at (3,6), considered to be the "north" cell. I prefer my solution since the line ends up at the "north" edge of the grid.]
Letters that extend beyond the printed grid are in bold. Added spaces are represented by the # symbol. Redundant letters in the wordplay are underlined.
Across
1. KNEADERS - def. "manipulative people", anag. of ASKED + N + R + E
6. SLURS - def. "insults", P for L in SPURS
9. FUSING - def. "Act of Union", creating a commotion = FUSSING
11. MANIA - def. "fanaticism", ROMANIA - R
12. NAC#RE - def. "shiny coating", hidden in LAMINA CREATES
14. OK#API - def. "browser", OK + API
16. AT EASE - def. whole thing, ATE + A + USE
17. TRIBALISM - def. "group loyalty", (BALT + IS) in TRIM
19. INC#UL#CA#TED - def. "brainwashed", anag. of CULT CAN DIE
21. CHILDCARE - def. "responsibility for young people", L and CHAR in CHIDE
24. VIRAGO - def. "Amazon", (RAG + O) in VIE
28. HIT#CH - def. "couple", HIT + CH
29. AZ#YME - def. "unleavened bread", anag. of (MAIZE - I + Y)
30. SPORT - def. "fun", OR in SPAT
31. PLAICE - def. "accompaniment for chips", anag. of SPECIAL
32. TASTE - double def.
33. EXORCIST - cryptic def.
Down
1. INFANT - def. "at early developmental stage", IN + FAN + T
2. NAS#TI#C - def. "unrelated to direction of stimulus", anag. of (ACTIONS - O)
3. DIRE - def. "dreadful", R in DIET
4. GENERA - def. "groups", GENERAL - L
5. OSMOTIC - def. "associated with diffusion of fluid", M in (OS + OTIC)
6. SAKE - def. "cause", treacherous person = SNAKE
7. ULN#A - def. "body part", hidden in BEAUTIFUL NANNY
8. TRAPEZE - def. "apparatus", (rev. of ART) + P + homophone of "ease"
10. G#AL#LO#P - def. "fast pace", OP after GALL
13. CIRCUIT - def. "way round", CI + anag. of (I + COURT)
15. ASHTRAY - def. "receptacle", (SHUT - U) in (ARRAY)
18. M#AL#ICE - def. "spite", LI in MACE
19. NOVI SAD - def. "Balkan city", VISA in NOD
20. LIGHTER - def. "one fires", BLIGHTER without first letter
22. DIALOG - def. "Trump's speech", rev. of (GO + LAID)
23. ENMESH - def. "catch", anag. of (HORSEMEN - OR)
25. R#OTI - def. "sandwich", EROTIC without first & last letters
26. ACRE - def. "division of land", ACT + RE
27. CZAR - def. "leader" - Z in anag. of ARCH
