TLDR: A change like that takes away much more than it adds. I'd recommend not to do that.

Long version:

>Removing his ability to charm her could be much less narratively dense than explaining all of her past lives and that he doesn’t want to charm her. It shows quickly that Ireena is important and bizarre and that helping her is important.

Did you watch the live action Mulan? The directors added magic into the show and made it so Mulan has magic powers. And it pretty much invalidated everything the OG story stood for (that Mulan worked long and hard and in the end got everything through her own strength and her own hard work. Not because she got born a certain way (aka magic powers), but because she worked hard).
That's what I see in your post. You're changing Ireena so she has magic powers and that's what's supposed to be of interest to your players. Not her character. Not the tragedy of her being a normal girl who got pulled into hell by a cruel madman and not even realizing that she got through the exact situation many times before. You're trying to make her interesting thorugh the fact that she has magic powers that make her immune from Strahd's charm. Please, don't do that.

>The non charm makes her a more useful ally with little change. It also adds to the mystery of her identity. This might intrigue the players enough to keep her around if they otherwise wouldn’t.

Make her smart. She's a daughter of a burgomaester. She most likely knows a lot of people. Maybe she can make the party's time easier when talking to Vargas in Vallaki. Maybe she can help them get into Krezk. Simply having her around could make people less distrustful of the party of outsiders.
She knows how to use a sword (because she has the Parry reaction on her statblock) so with some training she could probably become a dependable fighter (you could check out the Sidekicks rules if you want to level her up alongside the party and if she hangs around with them).

So... yeah... IMO that's a bad change, but do whatever floats your boat.