Everyone reacts differently to this sort of drug. My Dad is on them to help with agression and we've noticed very little in the way of side-effects, and he is on 75mg in the morning and 125mg at night! He slept a little better in the first week of taking them. The information leaflet that came with the tablets says that a maintenance dose of this drug can be as high as maximum 750mg per day, although many drugs are reduced by 50% for elderly patients. Even so we are nowhere near the maximum.
I am not sure about the cognitive decline bit, whether they are saying that it accelerates the disease or whether the drug just tends to make things a bit worse as a side-effect, possibly because it tends to make people a little slower and more drowsy.
All we can do is to trust the consultant.
If you continue to get unwanted side-effects then you should keep calling whoever prescribed it until you get your husband reassessed.
Unfortunately a lot of this has to be trial-and-error because the reactions are so individual.
My Dad has also been put on Exelon (rivastigmine) because the consultant considers that it is more beneficial than Aricept (donepezil) for people with behavior problems, particularly if these are underpinned by "psychotic" symptoms ie paranoia, excessive suspicion, aggression.
For myself, I would tend to trust a field consultant with experience rather than a limited clinical trial...
I would say that if a drug was found to accelerate the actual disease, rather than just accentuate the effects of the disease, then it would almost certainly be withdrawn for patients affected.