I see Maxim’s actions as completely consistent with his character. Any inconsistencies we perceive early in the text are due to a lack of understanding of the complex relationship Maxim had with Rebecca and Manderley.
Maxim is clear from the beginning that the narrator brings a refreshing and welcome change from his sombre mood. When they lunch together for the first time, he tells her of the effect she has had-
I’ve enjoyed this hour with you more than I have enjoyed anything for a very long time.
The reader, and the narrator, believes this to mean since his wife’s death. It is only when we realise that he was never happy while she lived that we understand fully what Maxim is trying to communicate.
He feels he has to uphold the values of upper-class English society, which is why he allows Rebecca to torture him. It would be, he believes, more painful to sully the family name with divorce and lose Manderley than to bear Rebecca’s taunting infidelity.