If you attack second, Nova will have a gap, but in the first place, he rarely does Nova Infinity if he attacks last in Cyber style
When Clives Hideaway hits the board, its owner looks at the top 4 cards of their library, picks a legendary creature from those cards, and tucks it away for safekeeping
There are plenty of other possible ways to use Purphoros, such as with token making, which pays you from both of his abilities
The Bident also gives you interaction that forces all of an opponents creatures to attack, hopefully leaving them wide open for you to swing for lethal commander damage
It sets up powerful plays, and its a reasonable card to mill since unearth gets you some value
Your options range from white recursion, to a bunch of utility artifacts, to a few good red cards that can do the job