Well, my reply may not be what you expect, but I'm going to be honest; I feel that "what is missing" is really a plausible explanation for the scenario being presented. In my opinion, your major flaw is that you are expecting the viewer to somehow almost telepathically acquire the long and somewhat-complicated explanation you've given for the scenario which you're trying to present. This is a common mistake of diorama builders--expecting the viewer to "know" what you were trying to portray without actually giving them enough elements to be able to make that clear.
I assume that reason you're saying that the Morser is "abandoned" is so that you don't have to put any figures into the scene, correct? In that case, all you have is a barely-damaged artillery piece next to a disproportionately-damaged and destroyed carriage. The average viewer isn't going to understand this instinctively; instead, your presentation leaves a lot of holes for the viewer to try to fill in and questions to then be raised.
If a crew had wanted to disable a Morser, they would have most likely placed demolition charges in the mortar's mounting trunnions--where the mortar mounts to its carriage--rendering the mortar un-firable.
A better presentation would have been to have the mortar intact, next to the destroyed carriage with crew members pointing to the skies and running around willy-nilly, showing that there was indeed a threat from the dreaded "jabos" imminent. This would explain the bombed carriage but the still-intact Morser, having been recently either lowered from the carriage or returned from a mission.
And one could go a step further and wonder why the Morser would not have been destroyed at the point of firing "a cpl of miles away" if the carriage had indeed been destroyed? Surely word would have been conveyed to the Morser crew by courier or radio that their transport was destroyed? Why labor back to the site if that were the case? You didn't show that all the crew was killed--there are no bodies on the ground? Surely there would have been guards or personnel tending the carriage while the Morser were in firing mode? What happened to them? See what I mean about "questions" being raised?
If I were to suggest making this dio more believable at this point, I would say to go ahead and "destroy" the whole thing--Morser and all, next to the wrecked carriage. Then at least the viewer looks at it and says "Oh, cool--a destroyed, bombed-out Morser!"--without having to mentally navigate through all those questions and assumptions that you're now expecting them to make.