I would imagine it would have been unlikely that Alexei would have lived long enough to marry even if the revolution had not have happened. He nearly died at Spala, and as he grew older it was harder and harder for Alexandra to stop him from injuring himself. As sad as that may be, I doubt he would have lived to see 20 regardless. Even so, I will join in the conjecture.
I doubt Missy would have wanted Ileana to marry Alexei and so I think she would have put a stop to any plans there. She adored Ileana, and marrying her off into the most unstable monarchy in the world would hardly have been her idea of a good marriage. Ileana would have been under constant threat and revolution was always a possibility in Russia. I can't see Missy willingly allowing Ileana to live in such a precarious state. Remember how anxious QV was about Alexandra marrying Nicholas? Everyone knew how dangerous Russia was.
I also think all Princesses coming from knowingly haemophiliac lines would have been ruled out. They knew that haemophilia was contracted from the mother, and the idea of having more haemophilia tainting the royal line would have been abhorrent to both Nicholas and Alexandra. So, there go most of the princesses directly descended from children of Queen Victoria.
I think Margarita and Theodora of Greece would have been the most likely contenders. Attractive, no haemophilia in their branch of the family, related but not too close, English in upbringing and I presume, Greek Orthodox, so no need for conversion? Plus they were minor Greek princesses and had no role as such in Greece, so they would have made the transition across to Russia fairly easily.
Though, knowing Nicholas and Alexandra, I think they would have, as far as possible, let Alexei make his own choice. They were so happy in their own marriage, so they wouldn't have wanted anything less for Alexei. Whoever he wanted, as long as they were suitable lineage wise, I am sure they would have been delighted to accept. However, seeing as we will never know what Alexei would have grown to be like as a man if he grew to be one at all, what his personality and interests would have been, etc, we can only make a limited guess based on who would have been suitable from a lineage point of view. Saying Ileana would have been the best choice based on a childish infatuation is a bit of a tricky point to argue, seeing as a childish friendship is very different from an adult marriage.
Rachel
xx