Will Almighty Allah hold one accountable for what one utters to oneself in anger at one's mother-in-law's actions, despite treating her well?

If you did not utter anything bad or curse the woman without justification, then there is no sin upon you. Nor is there any sin upon you for what you thought to yourself, because Allah has forgiven the Ummah of Muhammad (peace be upon him) for what they thought to themselves, so long as they did not speak or act upon it. You have done well by being patient with your husband's mother, as this is part of honoring your husband and having a good relationship with him.