Once Elisabeta (Nicholas sister) succeeds can she simply reinstate Michael and let him take charge - she doesn’t appear to really have much interest in it anyway
That would be an unprecedented move and very difficult as the Order of Succession is to the legitimate descendants of His late Majesty, King Michael of Romania, Prince of Hohenzollern in order of primogeniture.
Her Majesty Elisabeta Karina, Custodian of the Romanian Crown would then appoint an older relative (and his descendants) into the Order of Succession at the cost of her own position and that of her husband and their descendants (!).
The experience will be that once the sister is the Custodian of the Romanian Crown herself, and has all the wonderful properties and finances at her disposal, that she (and her husband) veeery quickly become accustomed and attached to that very unique position.
Nicholas Medforth-Mills should hope that his aunt Princess Margareta or her successor, his mother, Princess Elena re-instate him. Du moment his sister has become the Custodian of the Romanian Crown herself, the practical point of no return has already passed.
It would be the same as Crown Princess Victoria of Sweden fallen out of grace and out of the line of succession. Then her brother King Carl XVII Philip and sister-in-law Queen Sofia would abstain from the kingship by re-instating former Crown Princess Victoria, at the cost of their own selves and that of their descendants. Do you believe such a thing will
ever happen? Of course not.
It can only happen when former Crown Princess Victoria gets re-instated by her father. When her brother has reached the position of King, like Elisabeta Karina, the point of no return has already passed.
Nicholas should stop affiching himself as Prince, he should protest when others name him a prince. He should distance from double-tongued "monarchists" dissaproving the late King Michael and his daughter Princess Margareta for an own shady agenda. He should keep a low and humble profile and do his utmost best to be more than 100% loyal to his aunt and his mother, always defend them and always praise them as amazingly hardworking exemplaric royals with a heart for Romania and Moldova and hope for a reconciliation. That is the only option left for him.