I don't think that Samshin is her mom. She's a deity in Korean mythology (https://en.wikipedia.org/wiki/Samsin_Halmoni), so I don't think that in this drama at least, that she would be related to WJ. However, if it's a role that different ex-humans fulfill over time, then that might be possible.
He's not Death, but a Grim Reaper (https://aminoapps.com/c/korean_studies/page/blog/korean-folklore-characters-the-joseung-saja-grim-reapers/G5Mp_l4lHnu0kggkDMpak1dgDZ74k0DaJ51). He could possibly have been a principled official in his life. I think the crown prince was also supposed to marry Lord Kim's daughter, so he might have been doubling as his future father-in-law as well?
I think Yŏm-ra Daewang (https://en.wikipedia.org/wiki/Yama_(Buddhism)) is more of an overseer, so she may not be so involved in the nitty gritty.
I've really enjoyed the show so far. The story seems so simple, but it's got so much heart. Hope the finale will be good!