How often you up the dosage depends on how the plants are reacting to the feed. If the lower leaves start yellowing, or if they show signs of deficiency (and the pH is ok) then up the dose. If you start seeing nute burn on the tips, back off a little. With mine it's usually about every week for the first 5 weeks, then full strength for the last 3, but it depends on your strain, among other things.
The length of time the plant requires to flower is genetic, and can't be shortened. It can take longer if the plant is subjected to stress, but that doesn't help you.
12/12 is the correct light cycle for flowering, yes.
It's pretty late to start topping, plants don't appreciate stress during flowering, I'd keep an eye out for balls appearing if I were you. Not particularly likely, but be vigilant.
Topping is normally done during the early stages of veg, when the plant is around 5-6 nodes tall. You'd cut off everything above the second pair of nodes, then veg for a few more weeks before flowering, to produce 4 main colas. For more info on topping, see this guide
Uncle Ben's Topping Technique to get 2 or 4 MAIN colas