To the best of my knowledge faith mattered for the wives of the Tsar and the heir.
The wife of Grand Duke Konstantin Konstaninovich, Elizabeth Mavriekevna remained a Lutheran and their children though only princes and princesses of Russia were in line of succession.
Maria Pavlovna did convert to the Russian Orthodox faith later once her line came closer to the throne.
As for the heirs themselves i don't believe there has ever been a situation where an heir or heiress to the Russian throne was not of the Orthodox faith before the fall of the empire.
Could Maria Kirilovna keep her succession rights when she married the Prince of Leiningen ?
Was she asked/forced to relinquish them ?
Up to what point is religion an issue ?
AFAIK opponents to the Vladimir line always accused Miechen never to have converted, thus damageing her husband and son's claim.
I am not sure if Karl-Emich has converted or if such conversion is acceptable or deemed opportunist.
His first marriage was equal so his daughter (unmarried) could be eligible if there are not any religious issues.
Would his 3rd wife, a Grafin v.u.zu Egloffstein, be considered equal, after the notorious non-equality of wife #2 who costed him the Leiningen succession ?
Keep in mind, once Georgy becomes Head of the House he could declare his children, if any, as dynastic. Nikolai Kirillovich zu Leiningen could do the same if he inherits.
In any event, I think Georgy will have to clarify who his heir is when the time comes
Grand Duke Georgy Michaelovich is engaged to his Italian girlfriend. Marlene Eilers Koenig has an item on her blog about it.
After Rebecca converted to the Russian Orthodox faith and became Victoria Romanovna the egagement was expected.
Maria Vladimirovna has given her consent for the marriage but has also stated that the bride will be HSH princess Victoria Romanovna and for some reason will not be HIH the Grand Duchess Victoria Romanovna.
Wikipedia is already labeling the marriage as morganatic.
That means after Georgy the line of Grand Duke Wladimir dies out. Offspring from morganatic marriages are not eligible for succession.
With the bride being 38 and turning 39 and the wedding expected to take place in the autumn of 2021 the chances of offspring are limited anyway.
As his mother has no full siblings, the next line would be that of Maria Kirrilovna. She married the 6th prince of Leiningen and that might be an issue as it's uncertain if Leiningen would qualify under the rules of the Imperial dynasty as equal. Im not sure if Cyril Vladimirovich ever made a ruling about that.
In the case the marriage is considered equal her grandson Karl Emich , who already is a claimant to the throne as Nikolai Kirillovich, would be the senior line but he has the same issue as Georgy because his 2nd and 3rd marriages were unequal. His first marriage was to princess Margaritha of Hohenlohe-Oehringen and resulted in a daughter princess Cecilia (1988).
However his brother Andreas married princess Alexandra of Hanover and they have three children. The oldest son married a princess of Prussia but a morganatic one. His younger brother married an untitled woman.
Maria Kirillovna's second son married princess Marie Louise of Bulgaria but both their sons married unequal as well.
As did their father's younger brother.
Maria Kirilovna's daughter Kira-Melita married Andreas of Yugoslavia. Their oldest son Karl-Wladimir married unequal too but his brother Dimitri (1965) is still unmarried and is also of the orthodox faith. At 55 he is still a bachelor but he could be the one to rival his cousin's daughter Cecilia of Leiningen (not of the orthodox faith as far as i know).
In case the entire line of Maria Kirilovna is excluded than her sister Kira's line would end up with Georg Friedrich Prinz von Preussen, the heir to the thrones of Prussia and Imperial Germany....