YESMISSA:
Who is LJT ? i didn't think there would be another engagement for him, since he was punished? there that means that he could be in town but not in the palace? I thought he had orders not to come back. Or was it decided after 5 years??
I had also read in the comments that it was precisely because of FL's engagement to YS that ML decided to do everything to come back before weeding for winning her back?
LGT is Luo Ji Tong. CSS helped clear his name which is why he's able to come back and get an engagement. It only takes him 5 years because while he's at the border a lot of things keep coming up that he has to deal with. He still has the same rank as before it's just now he doesn't go by LBY anymore because everything that happened to his family and what/why he did what he did has come out thanks to CSS. Also, he doesn't come back because of the engagement that CSS has with YS. He was actually happy for her at first and was doing everything to protect her even when he was far away. It's just that when he comes back and he sees her for the first time after 5 years, he changes his mind. He realizes he can't watch her marry someone else/can't let her go.
Also, he doesn't actually plan to marry LGT. He agreed to the engagement just as a way to be able to come back from punishment. He was planning on calling off the engagement and marrying her far away afterward because he has some dirt on her.
At least this is what I got from reading the google translated version of the novel.