I don't think i can be of any help, but i have pretty much the same problem. Since i started some medication a couple of years ago, my hair has never been the same. I began loosing loads, mostly in the shower, esp when the conditioner goes on. This, coupled with colouring it too often (although hairdresser claims dying won't make your hair fall out) i was devastated to go from long thick hair to thin, dry horrible hair. I had to hair a fair amount of trim over the last couple of years, we're talking about 3" at a time.
This year I've cut back on the colouring this year, only had a couple of little trims & my hair isn't growing as much. Now, it seems it gets to a certain length and it then goes all dry and frizzy (its straight) and looks horrid at the ends.
I've tried the Lee Stafford hair growth stuff too. The Shampoo & Conditioner was terrible - really dried my hair out - i ended throwing it in the bin! The treatment isn't as bad, i've used in the past & i do think it actually works to a degree, however, it too dries my hair out.
Not sure about the anti-dandruff shampoo thing? My niece has really long, waist length hair, grown from a bob in about 3 years. She uses pantene shampoo (not a popular choice because of the silicones in it) she washes her hair everyday, BUT - she also uses a anti- dandruff once/twice a week or so - because she say its the only shampoo that gets rid of the hairspray build-up after a night out etc. Then again she is 18 & i'm nearing 40 so big difference!
I've used Herbal Essences shampoo & conditioner for split ends before, i don't recall it helping my hair grow faster, but my hair was longer then anyway.
I'll be watching this thread with interest, see if any ideas come up!