Well, re-reading some of the early SD letters, I'd say you can certainly read them as hinting at a fear of cannon/gun fire even before 1718, even though I didn't find a direct reference to one single incident. The relevant excerpts:
1715: 9th July. Fritz begged me to fire the little cannon you gave him so much that I gave him this pleasure; he was too pretty [joli - which I think might be used in a broader sense than just pretty, i.e. in the sense of acting pretty?] and was just jumping for joy. 13th July. I have the cannon fired for Fritz every day, in order to get him used to it. He is becoming very pretty and taller at the same time.
1716: 13th February. I had Fritz shoot with a small pistol. I hope, when you see him, that you find that he will not be a coward. 19th February. Fritz shoots the pistol quite nicely; he says he wants to surprise you when you come back.
1717: 10th May, Major Derschau and Brissen are having dinner with Fritz today and after dinner they will go to the park, where Hast will have Fritz shoot. [...] 14th May. Fritz goes hunting every day. 15th May. Fritz exercises every day and shoots the cannon and the rifle. 16th May. Fritz asked me to tell you that he is no longer a coward, that he is a brave boy and that he is already firing the big cannon. 20th May. I have found a way to change Fritz, whom I will tell you on your return. 21st May. I knew how to make a small fort for him, where he will fire his cannons; it's at the park. He was very happy to be King of the Schützenplatz; it was Münchhausen who shot for him. 23rd May. Fritz says he wants to give you a treat when you get back from what he's been hunting; it will be in a small hut of planks, which I had him made. 28th May. Fritz has so much courage that you will be surprised and maybe you just don't imagine him as he is. You will find him changed, he absolutely wants to give you a treat on your return, which you will not refuse. [...] 30th May. Fritz says you will be charmed to find him so brave; I think you will be happy with him. 6th June. Fritz is happy to surprise you and fire his cannons in front of you; He also wants to give you a Supper.
One of the footnotes also quotes from Wilhelmine's letters to FW - on May 8th, 1717: The second news is that my brother had yesterday afternoon, by order of our mother, his regiment in review. .. and that he made it do all the military evolutions; today he must exercise his artillery. Finally my dear Papa will be very surprised on his return to find instead of a little coward that he left behind, a brave man who is not terrified of anything. Also, May 15th: My brother does wonders, talks only about war and hunting and Mr. Duhan has him exercising every day.
Re: Fritz and fear of gunfire
1715:
9th July. Fritz begged me to fire the little cannon you gave him so much that I gave him this pleasure; he was too pretty [joli - which I think might be used in a broader sense than just pretty, i.e. in the sense of acting pretty?] and was just jumping for joy.
13th July. I have the cannon fired for Fritz every day, in order to get him used to it. He is becoming very pretty and taller at the same time.
1716:
13th February. I had Fritz shoot with a small pistol. I hope, when you see him, that you find that he will not be a coward.
19th February. Fritz shoots the pistol quite nicely; he says he wants to surprise you when you come back.
1717:
10th May, Major Derschau and Brissen are having dinner with Fritz today and after dinner they will go to the park, where Hast will have Fritz shoot.
[...] 14th May. Fritz goes hunting every day.
15th May. Fritz exercises every day and shoots the cannon and the rifle.
16th May. Fritz asked me to tell you that he is no longer a coward, that he is a brave boy and that he is already firing the big cannon.
20th May. I have found a way to change Fritz, whom I will tell you on your return.
21st May. I knew how to make a small fort for him, where he will fire his cannons; it's at the park. He was very happy to be King of the Schützenplatz; it was Münchhausen who shot for him.
23rd May. Fritz says he wants to give you a treat when you get back from what he's been hunting; it will be in a small hut of planks, which I had him made.
28th May. Fritz has so much courage that you will be surprised and maybe you just don't imagine him as he is. You will find him changed, he absolutely wants to give you a treat on your return, which you will not refuse.
[...] 30th May. Fritz says you will be charmed to find him so brave; I think you will be happy with him.
6th June. Fritz is happy to surprise you and fire his cannons in front of you; He also wants to give you a Supper.
One of the footnotes also quotes from Wilhelmine's letters to FW - on May 8th, 1717: The second news is that my brother had yesterday afternoon, by order of our mother, his regiment in review. .. and that he made it do all the military evolutions; today he must exercise his artillery. Finally my dear Papa will be very surprised on his return to find instead of a little coward that he left behind, a brave man who is not terrified of anything. Also, May 15th: My brother does wonders, talks only about war and hunting and Mr. Duhan has him exercising every day.