1) Taking it once probably won't do shit...taking it more than once is stupid...I wouldn't waste my time...I suppose if you take it enough, your estrogen will go out of control which will inhibit your T production, but this is rather silly...If you are hellbent on lowering your T levels, then there are other less fruity ways of doing it (there is an article on this site called Your Doctor Your Dealer that explains this).
Note: I do not advocate using that method for your first go round. There are many issues that can be explained by Low T, but could also be explained by something else. I would only recommened that solution if you have a proven borderline low T, have ruled out all other possbilities for your symptoms, and your doctor is old school and won't prescribe until you hit a "magic number" of lowness (but first I would advocate finding a new doctor). Basically, that is a last ditch resort, IMO.
2) Probably not significantly. In fact, by taking exogenous T, you MIGHT inhibit other bodily functions that are responsible for schlong growth that could make you end up with a slightly shorter willy. I think the overall effects would be negligible in either direction though.
I've noticed a strong correlation between Low T symptoms and ADD diagnosis. Were you diagnosed with ADD as a child or an adult?
When I first started feeling shitty, ADD was the first thing I started pursuing because a lot of the symptoms fit...I had a hard time concentrating on most things, brain fog, etc. This was eventually ruled out, and I ended up having testicular cancer which crashed my other hormones. So be aware of these correlations, especially if the ADD seems to have been onset later on in life.