I tried to follow the advice on here and keep things light and tell her that I didn't know the answer
Maybe you are a bit confused about keeping things "light" and telling her you don't know the answer.
If she thought you did not take her talk seriously, then perhaps you were trying to act too cheerfully while she was talking? Keeping things "light" is the over-all mood to try to have around the home, but if she approaches you with serious issues such as her willing to do anything to save the M,.....that's not the time to act "light" about the subject.
Whenever you realize you've stepped backward, just pick up and move forward again. Learn from your mistakes.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!