Personally, I think it's someone outside the Xu and the Luo Family, here is my theory:
1. Luo Madame always talked about how independent 11th is, how hard it will be to control her (early episode when talking to Yuanniang). Plus on top of that, Luo Madame knows, 11th's mother is her biggest weakness, therefore, there's no way she would take the risk to kill 11th's mother because with her mother being alive, it's way easier to control her.
2. The Xu family technically doesn't have any grudge against the Luo Family. At that time, nobody even knows she was 11th's mother, so if they wanted to take revenge for Lianfang, it doesn't make sense either because when they saw 11th's mother, they even said "is this your mother" therefore, how would they know who to kill.
I am waiting for the little monk, who said "I forgot to tell that lady something" then magically disappeared. Either he told his master and his master got scared so he went to hid him or he was killed off.
My biggest guess right now was her death was more "accidental" as in she witnessed something she shouldn't have. This person could be someone who has the authority or it could be a careless servant/killer who was working on a mission for somebody and he was caught, so he decided to kill Concubine Lv.
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On top of this, I want to know who killed Yuanniang, my biggest guess is Xu Lingyi's mother.