Hi Badietta,
I am sorry to hear of your situation, although it is a very familiar one to me. My wife, Sharon, was diagnosed nine years ago, at age 48, with either atypical frontotemporal lobe dementia &/or atypical Alzheimer's. Don't let the disagreement between the docs worry you, it is an academic exercise that has little effect on you or your husband. The management and outcome of both dementias are very similar. The fact that he has responded so well to Ebixa is great, and probably indicates that there is some Alzheimer's in the mix. Sharon was on Ebixa but showed no improvement, whether it slowed the progress or not there is really no way of telling.
You ask how one deals with the anger and unfairness, and how did this happen. Everyone deals with this differently. For us, there was no anger, we accepted it as one of those things that is outside of human control and we moved on to trying to enjoy what was ahead of us as much as possible, and squeeze whatever goodness we could out of the time we had left together. If Sharon had been brain damaged by a drunk driver, or a gunshot, or a falling plane, I would have been angry. For these are things that could be avoided if someone had done something different. But this, it falls into a fact of nature - the sky is blue, snow is cold, Sharon has dementia, grass is green. It does no good to question any of these, you might learn why, but you will not change them.
I wish you both well as you go down this path. You are not alone in this.