The monogamy specifically is for everyone. Any type of other potentially romantic contact is repulsive from someone she is not imprinted on. That is tested and proven when talking to Miri when the threesome situation is found out. It isn't Liam's titles, that is specifically stated as his would only make Lani more likely to imprint and choose him. We know Lani has a title related to family she stated such but did not go into details. Likely chapter 2 or later story will begin to go into details of how Lani is able to imprint on two people. It is decently likely that removing the "curse" will be a major plot point in the campaign if not an end game component of the campaign.
To my knowledge there is no conversations about her imprinting on Liam (there is specifically comments that she is imprinted so affection cat clan title it would effect only Lani). She is mentioned about the growth part and how much stronger she would be if she had started traveling with Liam at level 1 which is also applied to Lani which you might have misremembered. Either of us could go diving into the source code and find the interaction text though and read all perminetations of it to verify.
the file in question is morning_events.rpy
"
D "750%% to growth rate is absolutely unheard of for any other beast race title. And it literally only applies to Lani or Miri."
D "If Liam were to party with Lani starting at level 1, she would be one of the strongest forces alive today."
show bg profession_talk_mom_02
M "Yeah, whatever, that's swell. But it grants double affinity with all of the Mimi Clan."
M "Miri is already imprinted, so it doesn't really impact her, but Lani is vulnerable."
show bg profession_talk_dad_02"
" M "I don't know if the two titles multiply or stack, but it doesn't matter, there is no way that Lani can avoid falling in love with Liam."
D ".... Hmm... if that's true, we should contact Miri."
show bg profession_talk_mom_03
M "...... But what if Miri meets Liam... what if they..."
show bg profession_talk_dad_02
D "... Really? Is it something that could even turn off the tribal instincts?"
show bg profession_talk_mom_03
M "I don't know! It's... It's... I just don't know."
D "... Well, I'll go ahead and send a message to Miri, then."
show bg profession_talk_mom_02
M "Thanks... that makes me feel a little better...""