To address the questions you have...
The brother, for unknown reason, is cut out after FL finds out the truth about his disappearance. I think the novel does explain what happened to him after FL finds out. He was cured of his curse by a doctor (which never happened here) as well as the rest of the servants and Clodan (not sure about him).
ML became greedy for power because he was raised without learning the important values. His birth parents died early before they could really raise him. His godfather, a sorcerer in truth, probably didn't expect him to become selfish. He figured it was no harm to teach the knowledge of magic.
When his godfather found out he was researching immortality (hence ML's greed), he tried to dissuade him but ultimately gets betrayed and stabbed in the back by ML. With his remaining strength, he cursed ML to freeze his time (immortal) and filled with lust in exchange of never being able to leave the mansion. This is to prevent him from do bad things like let's say conquer the land, ruling, blabla and etc.
ML didn't want to be alone so he cursed his servants as well. They could leave anyway but their appearance will transform into a monster like they've been trying to hide it during FL's stay.
ML never lost his memories. It was a ploy to win back FL's heart again after he regretted all the things he did to her.
As of how he broke the curse, it was letting go of his magic talent. He had to give up everything he sought for: power.
Plot wise I feel like it was so interesting, I was actually looking forward to see how this curse is ever gonna get lifted especially with how the brother got involved I had hopes that surely he'll get some sort of redemption, admit to his flaws and apologize to fl, just for it all to go down the drain. A lot of the main plot points felt like they got cut short and the sex scenes just became more and more irritating as this went on.
I stopped at ch72 or smth because I cannot stand that after all this ml's memories are just wiped and this asshole never once apologizes to fl or any of the servants he tormented for years. Why did ml want to live forever? Why did his godfather master teach him magic and why did he curse him?? I still don't even understand what this curse even is did it just make him uncontrollably horny??? Is that it??? Why were other people dragged into this mess?? We knew from flashbacks that the dude has always been arrogant but be learns absolutely nothing??? How was the curse suddenly lifted?? Why does he get a happy ending in the end?? I don't mind toxic mls but this one was especially insufferable and the art is wasted on this douchebag
Of course the fault isn't completely on the ml because the fl is equally stupid and I cannot sympathize with her I'm sorry. I cannot stand her throughout all of this.
I'm not finishing this I simply refuse after the headache it made me go through and I feel it's just gonna give me more sex scenes I didn't ask for.