There are no horizontal word boundaries in the grid. Answers to "across" clues must be written on their correct row, in the order given, with no spaces.
Six "down" clues have had one letter moved from one word to another. This must be restored to its proper place before solving. The moved letters spell out a generic name for a member of a certain group. Furthermore, six "across" clues are missing a letter, which must be restored when solving. The restored letters spell out one specific member of the aforementioned group.
The first letters of all twelve "modified" clues mentioned above spell out the real name of a notable person who was better known by another name. This other name reveals two additional members of the group.
Once the grid is complete, some letters must be highlighted to form a name for the thematic group. For each across clue where a letter was restored to the nth word of the clue, highlight the nth letter of the answer in the grid. For each down clue where a letter was moved from the nth word to the mth word of the clue, highlight the nth and mth letter of the answer, as well as every letter in between.
Across
5. a) Softcovered container with head of polar bear moving inside
5. b) Author of East to West to retire... from east to west
9. a) For instance, mustard chicken
9. b) Retreating American soldier grabs a little BLT, a type of sandwich
10. a) Acquire retro Tesla, for example
10. b) Keratin rowing essentially makes small sound
10. c) Ameliorate Al, perhaps in possession of tourist on vacation
17. a) Pine of great size
17. b) Rome escaped sailor with pair of galleys? Hard to believe
17. c) Office gutted, evicted by Sylvester Stallone, having spoken to describe 20 down
19. a) Language spoken in the west of Africa and used in Microsoft Windows
19. b) Support for Laurel and Hardy's fourth
19. c) Lare cut in quiet criminal results in loud cry
24. a) Sick man thrown out of factory
24. b) People of Senegal about to eat tip of lion's tail
24. c) British tabloid enthusiast turned – he's caught inside (2 wds.)
28. a) Slime invasion's start: Top of the morning? (2 wds.)
28. b) Warmth and light from the skis of sportsmen from Arizona, Hawaii and Nebraska
Down
1. Electronic marker put up, showing entrance (4)
2. Perhaps do the butterfly favours, ending with extremities in industrialism (4)
3. Start of soldiers' three-wheeler attack (6)
4. Native American folk, ultimately (5)
6. Resort city for old Chess champion that is retired (5)
7. Cellist might take one before and after concert? (3)
8. Tightly gripped around body part (5)
11. Chess – essentially, board game for oneself (3)
12. Old roes sag too lobose, striking middle of toe (5)
13. Jets' estate, unfortunately, mostly pry to Lions? (5)
14. Trined blade that is abound by farm machine (5)
15. Yemenite taches collecting rain (5)
16. A bit of 28b across for Charles (3)
17. Biblical male officer briefly consumed cow's heart (3)
18. Musician, Reed player rocked just seconds (3)
20. In World of Warcraft, evil tree (6)
21. Corners of disapproval met with approvals (5)
22. Love Bleach – striking, epic coser showing Caribbean magic (5)
23. Member of thematic group needing clothes in festive city (5)
25. Marvel's Wolverine lost $1000, leading to debt (4)
26. Brown 19b across's sides scratched (3)
27. A plant, having cute alien leader, wanting core of modules or vessels (4)
