Part of me wonders if Zeus is having trouble because he's half-arsing the vow instead of giving it his all. He could have been a LOT more direct telling Susan about the rules for immortals changing but he feared reprisal more than fulfilling the vow.
We also shouldn't discount Adrian, who might be very willing to help a sane version of his mum (mom? Mam?). While it's not a route to raw power it's possible he can help Hope regain her skills quickly, which would help her mentor Tedd and Sarah.
As a side note, I assumed what Pandora tried to pass on that most Immortals don't is her emotional connections rather than her personality.