Yes, looking back on it, your reading of the text is so spot on, I have nothing to add. It's easy as a reader to place more blame on the one who is having the affair that it becomes hard to view things more objectively. And Arnold truly let her down. It's tragic that this could all have been avoided through communication and trust.
I hope once it's properly translated into English, I can better appreciate the nuances that Google Translate can't express.
And yes, poor Lilian

. I'm so glad she gets a happy ending.