You are absolutely correct about the nominal length of a shotshell being its fired length, not its loaded length, & unless excessively long a shell longer than gun is chambered for can normally be loaded with no signs for detection. It should be noted though no shell should be used in which the loaded length of the shell actually enters the cone. This can cause a resistance in opening of the crimp & bring on a drastic increase of pressure. This is most apt to occur using roll crimp shells being about ¼" longer than chamber. Also many British "2½" guns will have chambers of 2 9/16" (65mm) length or even 2 5/8". I know you said you are not planning on firing this gun, but it does need to be pointed out for certain a shell, particularlly a fold crimp as most present day shells are, can be loaded freely in a chamber considerably shorter than the nominal length of the shell with no resistance to show it. This does not mean the "Should" be shot, but they quite definitely "Can" be.
