but I personally think she'll be given a position like a Guardian's subordinate or the " area guardian "

I don't know about that. The reason why some people think that she is going to end up being a breeding sow is due to several reasons. First, she is the Slane Theocracy's trump card. Once Ainz finds out that they were responsible for Shaltear's mind control, Ainz would fucking nuke the entire country, send every single Slane Theocracy citizen to the Happy Farm, and plunder every single Yggdrasil items from their vault for revenge and to ensure that the Slane Theocracy's world item will never harm Nazarick ever again.

Second, Zesshi's own mother had the unfortunate fate of becoming a breeding sow to the Elf King. It would not surprise me to see Zesshi becoming a breeding sow to an even more powerful elf like Mare. You know what they say...like mother like daughter.

The only way for Zesshi to come out of this without a bad ending is if Zesshi betrays the Slaine Theocracy, proves her undying loyalty to Nazaricks, and shows that she has some kind of skill that makes her worth keeping alive. This is assuming that Ainz doesn't get angry enough to not kill every single person in the Slaine Theocracy for what they did to Shaltear.