It's less sacrificing you and more attempting to see if Sothis emerges from the Creast inside your body. You wouldn't be alive without it, so you do kinda owe your life to Sothis and Reha in that regard. Judging from the fact that Byleth became the dominant personality after merging with Sothis, it probably wouldn't have even worked anyway.

It's not like she isn't regretful of the price she would pay to see her mother again, she simply needs someone to confide in and be equal to after losing most of her family to TWSITD, much like Edelgard in fact. Hell, she even reveals that she is planning on having you succeed her as head of the church (and goes through with it in every ending she survives), so it doesn't seem like she was planning on keeping up the experiments anyway either. You're just one last attempt before she steps down.

Yup. She only saved you at birth so she could use your body as a vessel for Sothis later. Hell, that's what Byleth's mom was meant to be, but since that didn't work out..