Noleman, I think you now have several issue's here. Before I start I should tell you that I am not an attorney and am no way trying to offer any legal advice, but rather my own opinion. I do think you should PM William3rd and ask for his professional opinion and options.
First, once the visa has been issued I believe you have 6 months in which you have to use this visa. If she does not use this visa during that 6 month time frame then it becomes void and it's like starting all over, although it should be a little quicker the second time.
I gather that when you started your K-1 your fiancée's son was 17 and he was included as part of the K-1 package. Now that he will be 19 or 20 after his time in the army he will no longer be part of the K-1 package. I think he will not be allowed to come at the same time when your fiancée can come, but instead once she is here, and probably only after being here for two years. At that time she would need to begin the paperwork to bring her son. Again William3rd will know the legal options here and I am only commenting on what I "think" the end result could be.
If your fiancée is not willing to leave Russia now because she wants her son to come, then I kinda doubt that she is going to leave him for the two years it will take for him to be able to recieve visa.