I have a big problem with the relationship between the players, Strahd and Ireena - mainly the fact he only needs to bite her once more to turn her, and the only thing stopping him is several level 3 player characters.

This dynamic makes no sense to me.

I have played through Curse of Strahd, and am now DMing it for a different group. During my initial play through my DM ran the game RAW, and it made very little sense to me as a player then, but as a DM I am struggling with this concept even more.

I have thought up a few different scenarios which help it make more sense to me (unless I've overlooked something massive) - I am also adding some bits from the MandyMod and DragnaCrata reddit posts. I really like the idea of Strahd being a redeemable character, and also immortal. The party won't know this until they kill him, assume the campaign is over only for it not to be.

• My first idea was that he had bitten her once/twice already, but the final bite, to turn her into a higher vampire like him has to be accepted willingly with a mutual exchange off blood or come at a specific time, like a blood moon which could occur once every few months. This gives him a reason to stay back for a while, and just toy with the party, to kill time until he can finish his plan.

• My second idea revolves around a fairly large over haul of the past narrative and comes in three separate flavours:
• First - Strahd was an only child, he adored his mother, and she adored him, after he defeated his families enemies and settled in Barovia he moved his mother to Castle Ravenloft, and commissioned a massive portrait of her, the artist wound back the years and painted her in her youth, a beacon of beauty. Upon seeing this she began to obsess with regaining her youthful looks and made Strahd swear he will do everything in his power to help her. So he travelled his new lands and found the Amber Temple, he was tricked by Vampyr into swearing a pact. During his travels he meet Tatyana and they fall in love. He returns home with her and his Queen Ravenovia flies into a rage and kills Tatyana, Strahd upon finding her kills and drinks his mothers blood. The pact takes hold, he becomes a vampire and the dark powers toy with him as they reincarnate Tatyana over the centuries, with no knowledge of her past lives, brought up on stories of the vampire dreadlord of Barovia who rules their lands. This drives Strahd to become colder and emotionless. He has tried countless methods to win over the different incarnations of his beloved but none work.
• Second - The second one is quite similar to the first except Sergei kills Tatyana, not Queen Ravenovia.
• Third - The final one is a mix of the original backstory, the beginning of Castlevania (the netflix anime). Strahd became a vampire after his brother stole Tatyana from him and married her, he killed him, drank his blood turned into a vampire, cursed Barovia to a demiplane but then he stops Tatyana from killing herself and turns her into a vampire. He forces her to marry him and they live together for a short time, in that time Strahd begins to realize he never loved Tatyana, only lusted for her, and now she is a vampire and refusing to drink blood she is deteriorating and warping into something he does not want. After a few years she is nothing but skin and bones and Strahd releases his hold on her and she died, but never resurrected. It was centuries later, when he met a woman who managed to somewhat thaw him icy deminer.  This is Marina, not born of Barovia, but an adventure who came to this land by chance, and caught Strahds eye. She settled in Bere, where Strahd visited her at Berez, and she at Castle Ravenloft. Once the Burgomaster and Priest of Berez found out about this they burned her at the stake before Strahd could intervene, he slew everyone at the village and then flooded it with the river, washing any remnants away. Ireene is Marina reincarnated and Strahd did not want to turn her originally and does not want to now. He only bite he once upon their first visit due to a lack of control.  

• My final one was to have Ireena flee from the Village of Barovia with the help Father Donovitch after the Burgomasters death. He took her to the outskirts of the Village and paid some Vistani to take her to Krezk. She then can either be captured by Strahd, or another NPC or found some shelter somewhere along the way. I think this works what ever option I take as you don't have to worry about - The players escorting a fragile NPC and no-one likes escort missions, it adds more tension as they are searching and looking for clues to her location, it gives flexibility to the narrative and they can't accidentally forget her somewhere or take her somewhere and she gets killed. I have also had one of my PC's be Ireena's blood brother (they don't know this yet) and Ismark will recognise him.

Mostly, what prevents Strahd from biting Ireena a third time is keeping her inside a building that Strahd lacks an invitation to enter. There's no reason to drag Ireena along on any adventures, there's just a single escort quest (get her to a safer town) which Strahd might not notice happening until too late, at which point you can toss her into a house and forget about her.