okay im not 100% sure but, for the first question, yes that black haired man is claude's brother and jeanette's father. or somewhat her father.
2. yes im almost positive he used the black magic for diana. but im not sure for what. it most definitely had something to do with saving her though.
2. lucas will get his memories back on athys 15th birthday.
Spoilers:
1. Yes and No...
He's definitely claude's big bro physically but the spirit inside is not athanasius ...
In the flashback, something called his bro in their yard which is probably the spirit of white haired wizard (who turned dark for some unknown reason) who Lucas is acquainted with (still a theory) cuz he knows who Lucas is and messed with his lab (which he and the white haired wizard only used)....
2.he did used black magic with diana and athy in mind BUT not for something like saving the life of diana... it's a spell to forget someone...
Bc he was in so much pain after losing diana over the child, he wanted to forget what diana looks like and the spell needed 6 yrs to be completed but he met athy at the age of 5 and remembered Diana's face through athy who was her spitting image that's why the spell was broken...
If he did met athy at age 9, then it meant that the spell was completed and that's why he was ok to ignore athy and didn't felt anything... Athy accidentally broke it when she wondered to Claude's palace claude is always reminded of diana whenever he's with athy...
3. Yes he would regain his memories...
The manhwa diverted the story line from the novel so no one actually knows rn how things would progress, but one thing's for sure.. claude will regain his memories and apologize to athy....
The real one with tragetic story is jeanette... After re reading the whole manhwa, she seems pitiful rather than being spiteful cuz of her naiveness... She desperately wanted a family, was deceived by her aunt, didn't know she was a chimera, was experimented as a baby and his dad literally came back to life but at the same time it's not her dad inside that body... But despite everything she was angelic ╮( ̄▽ ̄)╭
Ps. Still don't like her tho
Disguised as lucas?? I don't think so... since he clearly stated that he hate lucas (when he said lucas will never understand friendship or whatnot)... Probably used black magic to maintain his soul and attracted the first prince to use his body .... Still just a theory cuz that's the summary of what happened in novel... Dunno in manhwa tho

i have three questions
1. is that black haired shady guy claude's brother? since his eyes are not actually black but crystal as well and he's been said to be jeanette's father or am i tripping??? didn't claude kill him? what the hell is going on how did he come back to life ??
2. when did claude use black magic? do u think he attempted to use black magic to save diana?? since he knew she was gonna die by giving birth to athy? or did something happen to transfer the curse jeanette is supposed to have since she is made of black magic herself? damn
3. please tell me he's gonna regain his memories. i would be so heartbroken to see him not get them back oof i really hope they end up living happily ever after but for some reason i'm getting massive bad vibes why do i feel like jeanette's gonna go crazy or sumn anyways i don't know if the novel is actually over but damn im so curious