Einstein,
I'll try to answer your questions, with as much explanation as I can, from my limited knowledge, in order to avoid offending you.
Price in 2000 and Rushton earlier in 1996 determined that the use of topical Finasteride was nominally effective in the inhibition of DHT. If there were any such effects, it was because of systemic resorption (absorption); (which I believe, you are trying to avoid, by using the topical preparation)
So the effectiveness comes from the penetration of the drug into sub-cutaneous level and into the blood stream, i.e. systemic effectiveness.
I hope this helps and I hope my verbage was not harsh.