Not really, slavery hinders economic and technological development so no one really benefits from slavery.
Half-right, humans also reproduce much faster than Singers do.
Can it be repaid? And by whom?
As established in WoR none of the main cast had the power to end Parshmen enslavement.
Navani and Dalinar you could argue, but Kaladin and Shallan were both too poor to afford Parshmen.
Like what?
Nothing they can do about that, how someone is treated is the personal choice of the people they interact with.
Let's not go there.
You're fine.