] While Edward's reaction to his mother's actions may not be the same as Brandon's to his father's, Brandon does know that when a man is desperate, he is capable of anything. He, not really knowing Edward, has no idea how his desperation may manifest itself, nor does Edward's mother.

It's true that Brandon doesn't know Edward well enough to predict how he would act. However, Colonel Brandon must be aware that he (that is Brandon himself) is more romantic and a creature of true sensibility than the average man--or at least than the men he spends time with such as John Middleton, Thomas Palmer, John Dashwood, etc.

Perhaps the revelation of a secret engagement leads him to believe that Edward is more of his way of thinking. On the other hand, once the news comes out about the secret engagement, it is also know that it has existed for four years. Not having acted on it in that time does not seem so much like desperation. From what little Brandon knows of him, if Edward were absolutley certain of meeting this opposition from his family, and desperately in love with Lucy, it would seem like he might have acted before this time.

Given that, the colonel's remark makes me think of two things:
First, that too many characters in this story so far have been acting based on inaccurate assumptions about other people, and that there is always a danger in that.