Spoiler:
That's an intresting theory. Xion's face that resembles Kairi was a result of her being a memory of Sora about Kairi. Thus, making her face similiar to her...but what about Ven ? What does he had anything to do with Sora ?
Ven and Sora did meet at Destiny Island... I think some interview or stuff had stated that. Check wiki.
One thing that was strange is that why did anyone didn't see Ven except for Xigbar. Does having a strong memory of Ven was required to see his face under Xion's cloak ? Hm..
Need birth by sleep now ;_;