I believe this is PC-98 Alice.
NOT Youmu

Would it count as an error if Alice's reflection in Young!Alice's eyes is reaching out with the wrong arm?

Actually, it doesn't seem so.

Isn't the reflection correct? The image of the hand for example is in the correct place.

That's a long hand...

The reflection is correct. If you look closely into young Alice's eyes you can see that she is looking at the older version of herself. Thus the "long" hand belong the older Alice which obviously means her hand is going to be larger than the younger alice.

I am genuinely surprised only one person noticed the reflection is wrong.
If the reflection is correct here, then the older Alice has a backwards hand.