"You have a practiced apathy, Mr. Wayne. But a man who doesn't care about the world doesn't spend half his fortune on a plan to save it
. And isn't so wounded when it fails that he goes into hiding."
Miranda is talking about Bruce Wayne spending on...Batman ? To save the world ? Not the clean energy project ? Not the investment she also made ? The same investment that Lucius tells him made Wayne Enterprises loose lots of money ?
I agree with you that Bruce and Batman is sorta a ridiculous secret. After a while everyone would pick up clues. But the premise for your theory clashes with the text of the film. It's factually wrong.
See, if you funnel the entire R and D budget for five years into a fusion project that you then mothball, your company is unlikely to thrive.
The timeline is very clear