I am a DIL who got married a few years ago, and I just want to send you good thoughts and hugs. I've read this forum for a while now, but I've never chimed in. However, in hearing about your experience, I heard so much of what I went through and wanted to share my thoughts.
My husband and I were together about three years before we got married, and knew each other several years before that. Like you, my MIL was wonderful until we became engaged. She had lost her only daughter, so my husband and I thought it would be a wonderful bonding and healing experience for us all to plan the wedding.
Very quickly, she became very controlling and possessive of my husband. Suddenly, he didn't call enough, visit enough, etc. Like your fiancé, he was also in the military; as you know, finding time to do all these things while deploying, training, etc., can be incredibly difficult.
The details aren't important, but ultimately things got very tense and ugly with the in-laws before our wedding. It was so difficult because a) I saw how much it was hurting my husband; b) I so much wanted to please everyone, especially this new family; and c) I had envisioned this beautiful, wonderful wedding experience.
Ultimately, I smiled, tried to be kind to her despite her obvious unhappiness and most of all tried to make it a beautiful day for both my husband and me. In the end, I still look back and feel a bit cheated out of the pure bliss of that day (I was nervous and stressed out about being around her during the whole event). But, she was the one who showed her true colors (she looks visibly mad in all of our wedding pictures and several guests commented on her unpleasant demeanor afterward).
Some advice for your day and thereafter:
1) Enjoy it as much as you can. Try to concentrate on you and your amazing new husband and block any unpleasantness out (easier said than done).
2) This is more easily accomplished if you surround yourself with a fortress of people who love you. My bridesmaids and several family members knew about the situation beforehand and were charged with buffering interactions with her. Best. Wedding. Gift. Ever.
3) Be sure to come off being the bigger person. Smile, hug her, whatever it takes. Your husband (and everyone else) will see you acting with grace, and this will do wonders for your marriage.
4) Remember it's YOUR wedding. It belongs to you and your husband, and you are kind enough to let everyone else be apart of it. Do what makes YOU happy. If you can include some elements that make others feel good, that's awesome. But ultimately, it's your day.
5) There's a great book called "Toxic In-Laws." It helped me a lot. You may want to consider reading it.
6) Encourage your husband in his relationship with his mom, no matter how bad things get between the two of you. I encourage my husband to call his parents once a week (Sunday is check-in day for him). At the end of the day, she then cannot point fingers at you for the deterioration of their relationship.
I feel so much for your situation because I know how stressful all of this can be! I hope you and your husband can enjoy your day and, more importantly, the rest of your lives together! Maybe things will improve after the wedding ... I really think these events are triggers for the crazy to come out in some people!
Best of luck!