I don't understand why did they need to seal Alexander after sealing the saintess elf.
I might not be remembering this properly but this whole situation seemed different in the novel.
He needs to seal himself because he is using Taboo to save her. When he uses Ante's power to create a new Taboo, in this case it's something like having your original personality is not allowed. They had to use powerful Taboo magic instead of normal Domination spell because they were afraid the latter wouldn't remove any curses or brainwashing the dark elves might have installed in the Saintess as contingencies.
But Taboo not that convenient, its imposed on the caster as well. Even Ante will be affected if she casts the magic herself directly instead of the MC doing it.
The part that seems different is I think Demon Prince Jibalgias was supposed to be a temporary personality born in that place. It's the same thing as making the Saintess think shes a dog. It's not like she had a dog personality hiding inside beforehand. That's why "Alex" is confident that "Jibalgias" will do his job and take her out of that place properly, because he's a construct.
Though maybe the manga is spoiling a revelation that comes later in the novel, I might not have read far enough. The manga has already changed the order of many elements of the plot. Ante was worried in both versions about what "Jibalgias" might do and had the same countermeasures.