Spices are best at flame out or during a secondary phase. Boiling spices for long periods can give some undesired flavors. I've added mine at flame out or at secondary with very good results. Speaking of which my pumpkin will be ready to bottle in a another 2 weeks.
Half in none really. But on my IPAs stouts I lose about a half gallon. Cream ales and lighter beers I don't lose much. But this was an example for the OP. say he he filled shy of five gallons and had 1/2 to 1/4 trub loss. Could boost his ABV.
it will water out or lower your AVB if you miscalculate and add more water than desired..i may have done this once or twice but ive gotten good at it through out the years
I will be transferring the pumpkin to secondary this fri. not sure if I will add anything until I taste it...lot of trub and a krausen that's not dropping anytime soon it seems. beer looks and smells great though