If you were to go to a doctor for a stress test and found that there was some partial blockage of the arteries in your heart, would improving diet and exercise (along with quitting smoking, in my father's case) reverse this? What I'm mainly curious about is whether this type of fat/calcium deposit can be lost just the same as body fat.

It can be reversed by a life change. Diet, exercise... ~

Yes. What is needed is regular long cardio 45-60 minutes, at least somewhat intense, 5-6 days a week, and a much lower fat diet , and more fruit for anti-oxidants. The reversal process is slow, around 5% improvement a year. But that is far better than a more severe blockage.

But a stress test may not detect it. It did not detect my 99% blockages 9 months before I needed a triple bypass.

Body fat changes can occur much more quickly.

Once arteries are blocked and the heart muscle substains damage muscle, it will not heal back. Once any heart muscle is damage it will be for life. Doing the above allows to a much better life and longer life.If you recall in the old days they said rest, now they say move and do as much as possible.

Linus Pauling, who won the 1954 Nobel Prize in chemistry, advocated a regimen of vitamin C and lysine to reverse atherosclerosis.