@N.D. You seem to have a good insight as well as foresight! a rarity these days:) To answer your questions, CDECHS has its access from GT road and will not be managed by CDA at all. I think you are spot on in your decision of keeping three key variables in mind (Possession, maintainence, price gains) since you will start building later. A plot presently in a non-possession area will not become a well populated area in only 3 years.
B-17 is a great society, minus the distance. Its future will depend a lot on the Margalla avenue project, especially if you commute regularly to the city. You have 3 years, if you like it, you can take a chance.
Its true that AWT was developed quite some time ago, but you can never underestimate the "Khaki Factor" in this country, that is, when they have decided to get their act together (Now you do realize why the "Sangjani" Interchange had to be built inside AWT society!). AWT is completely surrounded by a boundrywall (as opposed to just the front), and also has some military installations to its south, so expect it to be like a future cantt. I have heard (not confirmed yet) that once house construction picks up, the army run CSD stores will be opening a superstore in its commercial area. If that also happens, my bet is on Atleast 3x price increase in next 4-5 years :)
At present, you can get a 1 kanal plot there for between 2.5 - 3.5 million depending on its location and block. D block is the most expensive, G & H blocks are the cheapest.
I also agree that Wapda Town might not be a good option in your timeframe.
Lastly, I would advise you again to visit all of the societies suggested by forum members while keeping your key considerations in mind. I am sure you will arrive at a good decision for yourself.