Follow up? Re diagnosis of mild ectasia (3.8x3.7 cm) on ascending aorta via cardiac ct: have had chest x-rays in past years (2010) with only normal results. Is this something an X-ray could miss?

Yes. The degree of ascending aortic enlargement is small and would no be expected to be picked up on a regular chest x-ray. A ct scan or MRI would be needed to reliably image a mild aortic enlargement of the size you mention. Follow up would be with repeat ct scan in 6 months and then yearly after that if no dramatic enlargement was seen in first 6 months. You should be on a beta blocker (medication).
Yes. Mild ectasia of the ascending aorta is almost impossible to pick up on a plain chest x-ray. You really need a cat scan for adequate follow-up.
X-ray. Can only see so much, aneurysms only dangerous after certain dilation, with other factors like hypertension.