Hi! My opinion may be irrelevant, but -in comparison to Parkinsons disease- in AD the pathology is quite diffusely distributed throughout the CNS (with certain preferences, of course). In contrast, in PD the disease focus is clearly localized in specific neurons in the S. nigra, which could be targeted e.g. by stereotactic injections to modify gene expression locally. For this reason implantation of embryonal tissue had a certain and lasting effect. Systemic gene therapy -if effective at all- can be expected to yield side effects, e.g. when it is based on interference with amyloid processing or due to the vector chosen. (Please note, that I am not an expert in this subject. I hope that you will receive responses from real experts).

most unlikely.  when clinically evident, the disease affects many structures within the brain. the cellular target is unclear (which neurons to treat?) and not localized. and the molecules to interfere with are not completely understood (what to target, abeta production, accumulation, degradation?). last, at the diagnosis, the pathogenic process has been going on for decades, according to recent reports, and therefore a number of alterations have already taken place that may be hard to undo.

The way I see it: Alzheimer's is a disease of old age. We are all doomed to suffer from Alzheimer's, just that some of us will die before it surfaces, thus "free" from Alzheimer's disease. As to what causes one to suffer the disease sooner than others, I am not sure. It could be accumulation of free radical that leaks from mitochondria, or environmental toxin, or slower clearance of neurotoxic amyloid, or what have you... 

Is gene therapy a possible treatment for Alzheimer's? Maybe, if there exist a gene therapy that can slow aging process and delay all age-related diseases (Alzheimer's, Parkinson's, cancer, arthritis, etc). Is it promising? I would say possibly. Caloric restriction research which has now led to research in SIRT1 & mTOR (both have relations to Alzheimer's), will probably lead to more research in other genes. No clinically useful application yet, but the progress in this field is fascinating. 

Please note that none of the work stated above is related to my research. Just some stuff that I read during spare time.

https://www.taconic.com/taconic-insights/alzheimers-disease/ad-gene-therapy-trial.html