I doubt
http://www.propecia.com has the answers to those questions, though it may have some information you can draw conclusions from.
First off, I read that DHT or at least testosterone helps hair growth during childhood, but that sometime after puberty it starts to hurt hair growth. I don't know if that is to different degrees in all men or if it still helps some men in adult hood.
Second, in most men, ALL hairs are hurt by DHT and probably other factors we don't yet know about. Hairs on the sides are not hurt nearly as much. Most people's sides will thin a little over their lifetime, and some people will hit NW7. The only constant is that the sides are much more DHT resistent.
Third, men on propecia lost hair after the second year. This is because genetics cause almost everyone's follicles to become more sensitive to DHT over time, so that whichever follicles are sensitive even to the low DHT levels you have on propecia will be the first to shrink, and more become sensitive each year. This is true of men who do not hav male pattern baldness. Even their hair counts drop slowly over their lifetime. Men loose hair on Propecia not because propecia looses its strength or causes sensitivity, but because propecia has finite strength, and male pattern baldness is driven by genetics and is a progressive condition, with increase sensitivity over time. There will be years when sensitivity remains constant, and times when DHT levels naturally drop and you actually grow a few hairs back naturally, but over the years hair counts will keep going down.
I think I already posted this picture, but here it is again. Notice the slow hair loss after the second year. This shows that Propecia only takes a year or two to regrow all the hairs that will thrive at the reduced DHT level. After that, the new loss if from more hairs becoming sensitive enough to DHT over time to shrink even at that level. Avodart saves even these hairs for a while, but if they are genetically programmed to be sensitive even to 10% levels, they will still go.
http://a248.e.akamai.net/7/248/430/2006 ... _chart.gif
All men loose hair over time. But men with male pattern baldness become sensitive faster. However, at the reduced DHT levels, the slope of that graph my be similar to normal hair loss.
Now as for your other question about growing more hair, the answer is it depends on your follicles. I strongly suspect that if some are sensitive to your current levels, then all will probably thicken at reduced levels. However, if they are not sensitve to DHT at all, and you just think you are thinning, then changing the DHT levels might not do much. You can even use minoxidil for a few months to jump start things and then get off it and not loose more than you currently have. At worst you will be back were you started.