Thanks all. It definitely is confusing. The question, however, is not whether it is acceptable to the house of Leiningen, but whether it is acceptable to the Imperial House of Russia... However, (to keep it somewhat simple), if nobility is considered sufficient for a spouse (for their children to be included), several others would be included in the line of succession. N.B. I also noticed that I mistakenly left out princess Marie Cecile in the previous version (but included her children).
LINE of VLADIMIR
1. Grand Duke George Mikhailovich Romanov (son of Head)
LINE of MARIA
2. Prince Karl Emich of Leiningen (first cousin once removed of Head; son of Emich Kyrill - married to Duchess Eilika of Oldenburg, himself the eldest son of Maria Kyrillovna Romanova - married to Prince Friedrich Karl zu Leiningen)
3. Prince Emich of Leiningen (only son of 2 by third wife countess Isabelle von und zu Egloffstein)
4. Princess Cécilia of Leiningen (eldest daughter of 2 by his first wife princess Margarita of Hohenlohe-Öhringen)
5. Prince Andreas of Leiningen (brother of 2)
6. Ferdinand, Hereditary Prince of Leiningen (son of 5 by princess Alexandra of Hanover)
7. Princess Alexandra Ehrengard of Leiningen (daughter of 6 by princess Viktoria Luise of Prussia)
8. Prince Hermann of Leiningen (younger son of 5; however, in unequal marriage, so any children he might have won't be included - using the same logic as George remaining a heir)
9. Princess Olga of Leiningen (daughter of 4)
10. Princess Melita of Leiningen (sister of 2; however, in unequal marriage)
11. Prince Boris of Leiningen (first cousin once removed of Head; son of prince Karl of Leiningen & princess Marie Louise of Bulgaria; himself the second son of Maria Kyrillovna Romanova - married to Prince Friedrich Karl zu Leiningen; in unequal marriage)
12. Prince Hermann of Leiningen (younger brother of 11; also in unequal marriage)
13. Prince Karl Vladimir Cyril Andrej of Yugoslavia (first cousin once removed of Head; son of princess Kira Melita of Leiningen & prince Andrej of Yugoslavia; herself the eldest daughter of Maria Kyrillovna Romanova - married to Prince Friedrich Karl zu Leiningen; in unequal marriage)
14. Prince Dimitri Ivan Mihailo of Yugoslavia (younger brother of 13)
15. Karl Friedrich, prince of Hohenzollern (first cousin once removed of Head; son of princess Margarita of Leiningen and Friedrich Wilhelm, (hereditary) prince of Hohenzollern; herself the second daughter of Maria Kyrillovna Romanova - married to Prince Friedrich Karl zu Leiningen; married to countess).
16. Alexander Friedrich Antonius Johannes, Hereditary Prince of Hohenzollern (son of 15)
17. Princess Philippa Marie Carolina Isabelle of Hohenzollern (eldest daughter of 15)
18. Princess Flaminia Pia Eilika Stephanie of Hohenzollern (middle daughter of 15; married to Baron Károly von Stipsicz de Ternova)
19. Princess Antonia Elisabeth Georgina Tatiana of Hohenzollern (youngest daughter of 15)
20. Prince Albrecht Johannes of Hohenzollern (younger brother of 15; in unequal marriage)
21. Prince Ferdinand of Hohenzollern (youngest brother of 15; married to countess)
22. Prince Aloys Maria Friedrich Karl of Hohenzollern (eldest son of 21)
23. Prince Fidelis Maria Anton Alexis Hans (youngest son of 21)
24. Princess Victoria Margarita Sieglinde Johanna Isabella Maria (only daughter of 21)
LINE OF KIRA
25. Georg Friedrich, prince of Prussia (first cousin once removed to head; son of Louis Ferdinand, hereditary prince of Prussia and countess Donata of Castell-Rüdenhausen; himself a son of Kira Kirillovna Romanova and Louis Ferdinand, prince of Prussia)
26. Prince Carl Friedrich of Prussia (eldest son of 25)
27. Prince Louis Ferdinand of Prussia (second son of 25)
28. Prince Heinrich of Prussia (youngest son of 25)
29. Princess Emma of Prussia (only daughter of 25)
30. Princess Cornelie-Cécile of Prussia (sister of 25)
31. Prince Christian-Sigismund of Prussia (first cousin to head; 4th son of Kira Kirillovna Romanova - married to Louis Ferdinand, prince of Prussia; married to a countess)
32. Prince Christian Ludwig Michael Friedrich Ferdinand of Prussia (only son of 31)
33. Princess Irina Maria Nina Kira of Prussia (only daughter of 31)
34. Princess Marie Cecile of Prussia (first cousin to head; eldest daughter of Kira Kirillovna Romanova - married to Louis Ferdinand, prince of Prussia; previously married to a duke Friedrich August of Oldenburg)
35. Duke Paul-Wladimir Nikolaus Louis-Ferdinand Peter Max Karl-Emich of Oldenburg (son of 34; son of princess Marie Cecile of Prussia and Duke Friedrich August of Oldenburg; in unequal marriage - his wife is the granddaughter of Karl, 8th Prince of Löwenstein-Wertheim-Rosenberg)
36. Duchess Rixa Marie-Alix Kira Altburg of Oldenburg (eldest daughter of 34; in unequal marriage)
37. Duchess Bibiane Marie Alexandra Gertrud of Oldenburg (youngest daughter of 34; in unequal marriage)