I posted somewhere about shampoos...the keratin isn't what keeps your hair relaxed, so stripping it away doesn't really do much. I've used harsh clarifying shampoos, including a shampoo that has sea salt crystals in it--you know, the alleged kryptonite to BKTs?--my only problem with that particular shampoo was the high oil content. I don't know the exact chemistry behind how BKT relaxes the curl, but my nearly 4-yr experience has taught me that using regular sulfate-based shampoos won't cause your curls to revert. The BKT formula contains silicone, and I believe this contributes greatly to the silky and shiny post-BKT hair, which sulfate-based shampoos will definitely wash away. There's no way around it...you're going to have to experiment, and you will have your own unique experience with the BKT.
That said, I don't use sulfate shampoos on a regular basis. I have used them several times in close succession even, and my hair is always fine. I used a harsh clarifying shampoo just the other day, and it was the best hair day I'd had in awhile. Perhaps you could just concentrate on keeping the dandruff shampoo on your scalp...perhaps coat the hair shaft with conditioner before you apply the dandruff shampoo to your scalp.
I don't know how absolutely important that 450˚ is to the process. For all I know, my iron doesn't truly reach that, regardless what the dial says. Try it and see what happens. If you're not satisfied with the result, consider buying a more powerful iron. Personally, I think it's probably fine...just make sure to do several passes and get that "smoke" rolling.