This is how I would explain it (though I may sound condescending). She was intelligent and competent, but her judgements were emotional. She came to China originally as a missionary. When she was young, she accepted the authority of her church. Then she rejected it and became an opponent, a reformer. But when she was old, she accepted authority again.
I think we must imagine her, as she grew older, mixing with people, especially Chinese people, who were imbued with the belief that it is proper to obey authority, proper to play one's part in maintaining authority, improper to question authority beyond a certain point, proper to maintain social order and stability. Imbued with the same beliefs, she had the passion of a convert (or the passion of a missionary). By the time the British introduced political reforms, she was frightened of them.
And which authority? Her associates, especially her Chinese associates, thought that it would be in their interests to serve the authority of the future, the People's Republic, and - they weren't only self-interested - they believed it was in HK's interests. I don't believe Elsie Tu was self-interested (though she was probably flattered and coaxed in their subtle way by Chinese officials) and she too believed she was acting in HK's interests. So it goes.