I can accept her leaving almost-adult Falco behind with the logic that he can take care of himselfPretty sure them getting together was obvious, she is an adult, not a child. And in this world people live hundreds of years so a 20 year age gap is nothing. It is like a 2 year gap.
As for Falco being abandoned, while she isnt the best mom, in this world 15 is age of adulthood. She didnt abandon a child, she abandoned an adult. The biggest issue is she is bad at reading people's emotion and thought Falco didnt care and she taught him everything so she moved on.
What I cant forgive is her emptying out their shared bank account, which is essentially your mom stealing your wages in a family business