Could echocardiography find a congenitally weakened part of the aortic sinus before it develops into an aneurysm?

Not likely: Echocardiography can be used to follow patients at risk for aortic aneurysms if they are at risk because of genetic conditions. The presence of an aortic valve anomaly may raise suspicion, but until an aneurysm begins to form, where it will develop is only an educated guess.

Hard call: Depends on obesity, prior surgery, technique etcetera. If weak but not enlarged it may not be visible at all. If seen ok, if not a ct with contrast could show enlargement.