This is a themed cryptic crossword, with six answers following the theme. For further instructions and details, scroll past the crossword and clues.
ACROSS
1. Without everyone, head off in area where nature thrives (6)
6. Defense one's intruding in one country (5)
7. Pythagoras calculated portion for knave (6)
8. Clear and essentially staggering landform (5)
10. Show's bringing in singer's first group (5)
12. Report: "Constellation's hiding" (4)
13. Civilization's losing one million in a single season? (5)
14. Heard a tree and a well (4)
15. Grieve for some of them (our neighbors) (5)
17. Criticize Sting's energy (7)
18. A strong point for the removal of hotel (5)
19. Record in Vivaldi's chamber (4)
20. Exercising in some form of ingenious cycling (5)
22/26D/5D. Extra-rare loin meat, if niece is upset (4-2-1-8)
23. Even parts are switching in revolutionary Russian textile (5)
24. Fierce bird near the tip of Cyprus (5)
25. Weapon of ancient deity: a pillar (6)
27. Essentially principal Australian islands (5)
28. Strongly criticize leaders of Aegean Sea voyage (6)
DOWN
1. Forceful insurgents initially rebuke, showing a form of resistance? (9)
2. Ethnic group (minor) last found west of America (3')
3. Open areas with small steps (6)
4. Perhaps West's group of vocalists (3, 9)
5. (22A/26D/5D)
8. Arab country and Berber state form impromptu plan... ends up getting seized by King Albert VII? (5, 7)
9. Dream dish, with sweet filling, returned and left out from plate (8)
11. Doing permanent damage to cars drifting on circular course (8)
16. Fool turns back on mythical creature nearly having a lack of power (2, 7)
17. Stand up and tease: "In the end, fifty-percent of Swedes chatter endlessly" (8)
21. Collects (bit by bit) German spare parts, ultimately (6)
26. (22A/26D/5D)
The check mark will go to the first person who submits an answer with: 1) an image of the completed crossword, 2) explanations for all clues, and 3) explanations for the theme and its corresponding clues. There is a relevant easter egg, but it is a more minor detail and need not be pointed out by the answerer to get the check mark.
Users who cannot use Imgur are exempted from fulfilling the first requirement, but it is highly encouraged for such a user to reproduce the grid visually in another way (preferably code formatting for consistent spacing).