1.) Yes, there are quality ADS devices that do work. Some work better than others, and I suggest doing searches to find the good ones.
2.) I think it is possible to gain a little girth while using an ADS, but it is primarily considered a length gaining device.
3.) I think you may be confusing extenders with ADS. While they both do the same job, most people do not put them in the same category. I have never seen what I consider an ADS cost $300 dollars, but I have seen extenders that cost that much or even a bit more. If you are looking to save money, you can find lots of ADS on ebay for less than $20.
4.) Remember that most members use an ADS as a supplement to a manual PE routine and not a substitute for exercise. A 2 inch gain does seem like a bit of a stretch, but most guys stop before they really reach what is still possible— so who knows. How long it would take depends on too many factors to predict, but I wouldn’t plan to hit a two inch gain in any less than 1.5 years.