Hey, I had SD on my scalp almost 80% fully covered. Now I'm completely clear and have been for months. (we're also same age, i had since last feb (2013))
Looking at your methods, we've used more or less the same things.
1 -Alcohol - Use Vodka. Definitely nothing with high yeast
2 - Black Cumin - I used this, read a lot on inspire.com about it a while ago, didn't really help, doesn't smell nice and takes months before seeing any form of results. There's much better stuff out there. Try it though; it may work for you. Really cheap I found too.
3 - Grape seed- I've seen that a lot around here too, sadly can't comment on it as ive never tried.
4- baking soda worked well for my forehead, which i've got a few spots left on. not too sure regarding scalp
5- milk of mal- Get milk thistle if you're going to do Liver Cleanses
too. Also remember to do colon cleanses, a simple colon aloe vera cleanse for a bit would help
What worked for me and cleared up major amounts was taking a probiotic supplement. what I used was a Yakult supplement (http://www.yakultusa.com/
or if you're in another country just google yakult)this cleared up the major majority of my skin.
My groin, scalp, forehead, back was all majorly improved; only spots that aren't cleared are the forehead with only about 3x2cm spots left, and i have a spot on my back which is 1cm i'll probably use some steroid cream to rid that.
Yakult is 7.5billion good healthy bacteria per bottle, i was drinking 4-5 a day for a weekend and it improved that fast.
Now, after finding out from another member a few days ago i would really not suggest that but rather "VSL#3" it contains 450 BILLION per sachet which is definitely amazing. Quite expensive but if you're willing to put the time and money into it now you'll be clear.
I strongly suggest changing the diet up, not too majorly, but if you're eating, say, a lot of junk food e.g. biscuits, cakes, fast food per day or week then cut it down and choose some fruit and veggies instead. You don't have to permanently be on it - but change it until you're satisfied with the results.
sidenote: my scalp gets maybe 1-2 flakes if i rub it. other than that, completely clear. I used to have 3cm thick plaques, yellow dead skin and when i combed it i could get like 5cm shards of dead skin at times. Really disgusting.
Hope this helps, it's what worked for me, i know the pain at this age, hit me up with a message if you ever need too.