Cryptic Crossword: Parallel Universes

Question

This is my first post! Feel free to let me know of any points of constructive criticism you have.

Cryptic Crossword: Parallel Universes

What you see before is not one puzzle, but four: each from an alternate universe. Although the machinations of destiny have caused each of them to differ greatly from one another, a keen eye may find they are not so different after all! To not spoil the fun, I'll only give you one such similarity: the letter on the yellow tile is the same in each puzzle.

Across:

1. Endlessly-spinning ring makes an exclamation. (3)
4. Good fellow to dry skin. (4)
7. To satisfy angrily would be to make twice of a vegetable. (3)
8. A choice surgeon's place of work. (2)
9. A piece of a broken biscuit? (3)
11. Draws from a complicated past. (4)
13. Orbital extremes befuddle libraries' primary source. (5)
14. Animal fat is the residue of a partnership song starting with sulfur. (4)
15. Dictator's ending is an obstacle. (4)
18. An ancient Egyptian, without their leader, turns to a telescope. (5)
22. A backwards half-divide flips out. (4)
24. The burnt remains of a hidden stash. (3)
25. Technician's pronoun. (2)
26. Soak Oscar in some pasta, initially. (3)
27. Stage signal with Romeo - that's the solution. (4)
28. I heard it before, what is inside the heart. (3)

Down:

1. A whale's song, butchered and unending. (4)
2. The operator takes their shot initially, making a decision. (4)
3. Snacks discovered in a sweatshop. (4)
4. Beds with energy become shelters. (5)
5. Oddly, half of the herbalists have a medical plan. (3 abbr.)
6. Constants found in the epistimology. (3)
10. Masyu puzzles conceal their direction. (2)
12. Edgar's profession cut short. (3)
16. Spinner's tale of prosperity, in brief. (3)
17. Follow-up to 9A: a salve. (5)
18. Beheading charge, taking over. (4)
19. Record-breaking pet overshadows the alpha. (4)
20. Being equal parts of vision, wisdom, and charisma. (2)
21. Singer offers a hearing. (4)
22. Californian Amedee code: a brief moment of eureka without the middleman. (3 abbr.)
23. Norse goddess' successor was beheaded. (3)

Bonus Challenge

As you solve the crossword, you may spot two more patterns shared between the different parallel universes. Finding them will allow you to answer the questions below (the questions themselves are not cryptic clues, but the answer format in the parenthesis follows the same convention):

Whose job is it to write beautiful words? (1,4'1)

And what would you need to do to have it sung to a large audience? (3,2,1,5)

Edit (08/05/2021): Fix to 5D and 20D

Answer 1

Missing the explanations for a couple of clues, but I think this is the finished grid:

Across

1. Endlessly-spinning ring makes an exclamation. (3) OHO?
4. Good fellow to dry skin. (4) CHAP, double definition
7. To satisfy angrily would be to make twice of a vegetable. (3) PEA?
8. A choice surgeon's place of work. (2) OR, double definition (a choice, operating room)
9. A piece of a broken biscuit? (3) CUT, picking three letters from "biscuit"?
11. Draws from a complicated past. (4) TAPS, anagram of "past"
13. Orbital extremes befuddle libraries' primary source. (5) APSIS?
14. Animal fat is the residue of a partnership song starting with sulfur. (4) SUET, changing the first letter of "duet"
15. Dictator's ending is an obstacle. (4) STOP?
18. An ancient Egyptian, without their leader, turns to a telescope. (5) OPTIC, "Coptic" without the first letter
22. A backwards half-divide flips out. (4) APES, half of "separate" backwards
24. The burnt remains of a hidden stash. (3) ASH, part of the word "stash"
25. Technician's pronoun. (2) IT, double definition
26. Soak Oscar in some pasta, initially. (3) SOP, O inside the first letters of "some pasta"
27. Stage signal with Romeo - that's the solution. (4) CURE, R in "cue"
28. I heard it before, what is inside the heart. (3) EAR, substring of "heart"

Down

1. A whale's song, butchered and unending. (4) ORCA, anagram of "carol" without the final letter
2. The operator takes their shot initially, making a decision. (4) OPTS, "op" plus first letters of "their shot"
3. Snacks discovered in a sweatshop. (4) EATS, substring of "sweatshop"
4. Beds with energy become shelters. (5) COTES, E in "cots"
5. Oddly, half of the herbalists have a medical plan. (3) HRA (health risk assessment), odd letters of the first half of "herbalists"
6. Constants found in the epistimology. (3) PIS, substring of "epistimology" (should be epistemology, though?)
10. Masyu puzzles conceal their direction. (2) UP, substring of "masyu puzzles"
12. Edgar's profession cut short. (3) POE; "poet" without the last letter
16. Spinner's tale of prosperity, in brief. (3) TOP, first letters of "tale of prosperity"
17. Follow-up to 9A: a salve. (5) PASTE, double definition
18. Beheading charge, taking over. (4) OUST, "joust" without the first letter
19. Record-breaking pet overshadows the alpha. (4) TAPE, A inside anagram of "pet"
20. Being equal parts of vision, wisdom, and charisma. (2) IS, substring of three different words
21. Singer offers a hearing. (4) CHER, homophone of "share"
22. Californian Amedee code: a brief moment of eureka without the middleman. (3 abbr.) AHC, "ah!" plus C somehow?
23. Norse goddess' successor was beheaded. (3) EIR, "heir" without the first letter

Hidden pattern

Every quarter-grid uses the same set of letters!

The bonus

Using the same set of letters to answer these:
Whose job is it to write beautiful words? A POET'S
And what would you need to do to have it sung to a large audience? SET UP A CHOIR!