So, I've done a lot of testing on 4.4.2 and discovered that the Young Lady is spawn command /friend 9. She looks exactly like Yona, has no color definitions or clothes, cannot be interacted with and even though the mouse-over on the character says Young Lady, she still has Yona's character card. So, the character category is definitely still a WIP. What's more, Yona absolutely cannot get pregnant yet, so this also needs to be fixed first, before a Young Lady can be created.
As far as killing Death goes, I have now done it multiple times. I have created a raid camp, where I bring all my female NPCs (male NPCs or monsters are useless in this battle) and equip as many as possible with Shurikens. I put the Ent Queen on a pedestal that is 3.5 stair levels high (2 + 1.5), from which she does her ranged attack and healing. I have another very high tower, with a platform surrounded by stone walls and a flag in the middle, to which I teleport my NPC with decoy charm, seconds after the raid has started and then leave him/her there, while I teleport back down to the camp. I also have a total of 100 steak dinners set on tables, on this same high platform where I safely hide the target NPC (the game only counts the first 100 meals, so no use to set more). My male MC's weapon of choice is the chainsaw (very rarely dropped by the big Halloween raid ghosts - spawn command /get wp_chainsaw), which in its heavy attack mode (F), can be used to make multiple rapid zoom and scoot attacks on Death. No need for cheats or spawn commands.
Oh, and /friend 173 is a Mothman, lol
/friend 174 is the Servant of Death, and
/friend 175 is Death
PS: Careful, the last two still do aura damage, even though they are 'friends'.
I've also now finally solved the mystery of why the normal (faint 2) club is marked as useable by the Giant, when in fact it is the Giant's daughter /friend 149, that uses this club.