Questions about Yonah, gestalt and replicant, spoilers

So I have ben trying to piece together the story as you don't get much in the game. From what I understand the BS is caused when a gestalt relapses and can no longer merge with the replicant.

If that is accurate, how did Yonah Gestalt merge with her replicant?

Once they were merged why was she still acting sick?

Once they were merged why did sunlight destroy her soul?

Bonus question, why didn't soul Nier grab replicant Yonah sooner?