I don't see why there would be any benefit to leaving it on longer if you blew your hair dry with the PT in it.
It's all cooked and done!
With blow-drying there is still the problem of blow drying (heat and drying to skin and hair) so that's why there's oil in the recipe. I think the epsom salts are really optional. In the Aphogee, I believe it's there to help the liquid congeal with heat. Aphogee 2-step is super-runny.
But there may be some benefit to the epsom salts, I just can't find a reference to the chemistry for that which is credible.

I'll be doing this tomorrow, so I'll report back too!